Printing Java Collections Nicely toString Doesn t Return Pretty Output

Question

I wish to print a Stack lt Integer gt  object as nicely as the Eclipse debugger does  i e   1 2 3      but printing it with out    output     stack doesn t return this nice result   Just to clarify  I m talking about Java s built-in collection so I can t override its toString     How can I get a nice printable version of the stack

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Guava looks like a good option    Iterables toString myIterable

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If this is your own collection class rather than a built in one  you need to override its toString method  Eclipse calls that function for any objects for which it does not have a hard-wired formatting

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With Apache Commons 3  you want to call  StringUtils join myCollection

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In Java8    will prints each element line by line stack forEach System out  println     or    to print with commas stack forEach       ele  - gt            System out print ele

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With java 8 streams and collectors it can be done easily   String format Collection lt   gt  c      String s   c stream   map Object  toString  collect Collectors joining          return String format    s    s       first we use map with Object  toString to create Collection lt String gt  and then use joining collector to join every item in collection with   as delimiter

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most collections have a useful toString   in java these days  Java7 8   So there is no need to do stream operations to concatenate what you need  just override toString of your value class in the collection and you get what you need  both AbstractMap and AbstractCollection implement toString   by calling toString per element  below is a testclass to show behaviour  import java util List  import java util ArrayList  import java util Map  import java util HashMap   public class ToString     static class Foo       int i      public Foo int i    this i i         Override     public String toString             return  quot   i   quot    i    quot    quot               public static void main String   args        List lt Foo gt  foo   new ArrayList lt  gt         foo add new Foo 10        foo add new Foo 12        foo add new Foo 13        foo add new Foo 14        System out println foo toString            prints     i  10      i  12      i  13      i  14         Map lt Integer  Foo gt  foo2   new HashMap lt  gt         foo2 put 10  new Foo 10        foo2 put 12  new Foo 12        foo2 put 13  new Foo 13        foo2 put 14  new Foo 14        System out println foo2 toString            prints   10   i  10    12   i  12    13   i  13    14   i  14           Update Java 14  Mar 2020  Records are now a preview feature not requiring you to override toString   if your class only holds data  Records implement a data contract  giving public readonly access to it s fields and implementing default functions for your convience  like comparison  toString and hashcode  so once could implement Foo as follows with the behavior  import java util List  import java util ArrayList  import java util Map  import java util HashMap   public class ToString     static record Foo int i           public static void main String   args        Foo f   new Foo 10       System out println f toString            prints  Foo i 10       List lt Foo gt  foo   new ArrayList lt  gt         foo add new Foo 10        foo add new Foo 12        foo add new Foo 13        foo add new Foo 14        System out println foo toString            prints   Foo i 10   Foo i 12   Foo i 13   Foo i 14        Map lt Integer  Foo gt  foo2   new HashMap lt  gt         foo2 put 10  new Foo 10        foo2 put 12  new Foo 12        foo2 put 13  new Foo 13        foo2 put 14  new Foo 14        System out println foo2 toString            prints   10 Foo i 10   12 Foo i 12   13 Foo i 13   14 Foo i 14

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You could convert it to an array and then print that out with Arrays toString Object      System out println Arrays toString stack toArray

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Implement toString   on the class    I recommend the Apache Commons ToStringBuilder to make this easier  With it  you just have to write this sort of method   public String toString          return new ToStringBuilder this          append  name   name          append  age   age          toString         In order to get this sort of output       Person 7f54 name Stephen age 29    There is also a reflective implementation

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Be careful when calling Sop on Collection  it can throw ConcurrentModification Exception  Because internally toString method of each Collection internally calls Iterator over the Collection

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You can try using  org apache commons lang3 builder ToStringBuilder reflectionToString yourCollection

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I agree with the above comments about overriding toString   on your own classes  and about automating that process as much as possible    For classes you didn t define  you could write a ToStringHelper class with an overloaded method for each library class you want to have handled to your own tastes   public class ToStringHelper             instance configuration here  e g  punctuation  etc       public toString List m               presentation of List content to your liking           public toString Map m               presentation of Map content to your liking           public toString Set m               presentation of Set content to your liking                 etc      EDIT  Responding to the comment by xukxpvfzflbbld  here s a possible implementation for the cases mentioned previously   package com so demos   import java util List  import java util Map  import java util Set   public class ToStringHelper        private String separator      private String arrow       public ToStringHelper String separator  String arrow            this separator   separator          this arrow   arrow            public String toString List lt   gt  l            StringBuilder sb   new StringBuilder               String sep               for  Object object   l                sb append sep  append object toString                 sep   separator                    return sb append      toString               public String toString Map lt     gt  m            StringBuilder sb   new StringBuilder               String sep               for  Object object   m keySet                  sb append sep                 append object toString                   append arrow                 append m get object  toString                 sep   separator                    return sb append      toString               public String toString Set lt   gt  s            StringBuilder sb   new StringBuilder               String sep               for  Object object   s                sb append sep  append object toString                 sep   separator                    return sb append      toString               This isn t a full-blown implementation  but just a starter

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System out println Collection c  already print any type of collection in readable format  Only if collection contains user defined objects   then you need to implement toString   in user defined class to display content

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The MapUtils class offered by the Apache Commons project offers a MapUtils debugPrint method which will pretty print your map

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With Apache Commons 3  you want to call  StringUtils join myCollection

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Implement toString   on the class    I recommend the Apache Commons ToStringBuilder to make this easier  With it  you just have to write this sort of method   public String toString          return new ToStringBuilder this          append  name   name          append  age   age          toString         In order to get this sort of output       Person 7f54 name Stephen age 29    There is also a reflective implementation

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Just Modified the previous example to print even collection containing user defined objects   public class ToStringHelper        private  static String separator     n        public ToStringHelper String seperator            super            ToStringHelper separator   seperator              public  static String toString List lt   gt  l            StringBuilder sb   new StringBuilder            String sep               for  Object object   l                String v   ToStringBuilder reflectionToString object               int start   v indexOf                   int end   v indexOf                   String st    v substring start end 1               sb append sep  append st               sep   separator                    return sb toString               public static String toString Map lt     gt  m            StringBuilder sb   new StringBuilder            String sep               for  Object object   m keySet                  String v   ToStringBuilder reflectionToString m get object                int start   v indexOf                   int end   v indexOf                   String st    v substring start end 1               sb append sep  append st               sep   separator                    return sb toString               public static String toString Set lt   gt  s            StringBuilder sb   new StringBuilder            String sep               for  Object object   s                String v   ToStringBuilder reflectionToString object               int start   v indexOf                   int end   v indexOf                   String st    v substring start end 1               sb append sep  append st               sep   separator                    return sb toString               public static void print List lt   gt  l            System out println toString l                  public static void print Map lt     gt  m            System out println toString m                  public static void print Set lt   gt  s            System out println toString s

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There are two ways you could simplify your work  1  import Gson library  2  use Lombok   Both of them help you create String from object instance  Gson will parse your object  lombok will override your class object toString method   I put an example about Gson prettyPrint  I create helper class to print object and collection of objects  If you are using lombok  you could mark your class as  ToString and print your object directly    Scope value    prototype    Component public class DebugPrint lt T gt       public String PrettyPrint T obj         Gson gson   new GsonBuilder   setPrettyPrinting   create          return gson toJson obj           public String PrettyPrint Collection lt T gt  list         Gson gson   new GsonBuilder   setPrettyPrinting   create          return list stream   map gson  toJson  collect Collectors joining

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If this is your own collection class rather than a built in one  you need to override its toString method  Eclipse calls that function for any objects for which it does not have a hard-wired formatting

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Newer JDK already has AbstractCollection toString   implemented  and Stack extends AbstractCollection so you just have to call toString   on your collection      public abstract class AbstractCollection lt E gt  implements Collection lt E gt                public String toString             Iterator lt E gt  it   iterator            if    it hasNext                return  quot    quot                StringBuilder sb   new StringBuilder            sb append               for                    E e   it next                sb append e    this    quot  this Collection  quot    e               if    it hasNext                    return sb append      toString                sb append      append

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most collections have a useful toString   in java these days  Java7 8   So there is no need to do stream operations to concatenate what you need  just override toString of your value class in the collection and you get what you need  both AbstractMap and AbstractCollection implement toString   by calling toString per element  below is a testclass to show behaviour  import java util List  import java util ArrayList  import java util Map  import java util HashMap   public class ToString     static class Foo       int i      public Foo int i    this i i         Override     public String toString             return  quot   i   quot    i    quot    quot               public static void main String   args        List lt Foo gt  foo   new ArrayList lt  gt         foo add new Foo 10        foo add new Foo 12        foo add new Foo 13        foo add new Foo 14        System out println foo toString            prints     i  10      i  12      i  13      i  14         Map lt Integer  Foo gt  foo2   new HashMap lt  gt         foo2 put 10  new Foo 10        foo2 put 12  new Foo 12        foo2 put 13  new Foo 13        foo2 put 14  new Foo 14        System out println foo2 toString            prints   10   i  10    12   i  12    13   i  13    14   i  14           Update Java 14  Mar 2020  Records are now a preview feature not requiring you to override toString   if your class only holds data  Records implement a data contract  giving public readonly access to it s fields and implementing default functions for your convience  like comparison  toString and hashcode  so once could implement Foo as follows with the behavior  import java util List  import java util ArrayList  import java util Map  import java util HashMap   public class ToString     static record Foo int i           public static void main String   args        Foo f   new Foo 10       System out println f toString            prints  Foo i 10       List lt Foo gt  foo   new ArrayList lt  gt         foo add new Foo 10        foo add new Foo 12        foo add new Foo 13        foo add new Foo 14        System out println foo toString            prints   Foo i 10   Foo i 12   Foo i 13   Foo i 14        Map lt Integer  Foo gt  foo2   new HashMap lt  gt         foo2 put 10  new Foo 10        foo2 put 12  new Foo 12        foo2 put 13  new Foo 13        foo2 put 14  new Foo 14        System out println foo2 toString            prints   10 Foo i 10   12 Foo i 12   13 Foo i 13   14 Foo i 14

User · Answer

Guava looks like a good option    Iterables toString myIterable

User · Answer

In Java8    will prints each element line by line stack forEach System out  println     or    to print with commas stack forEach       ele  - gt            System out print ele

User · Answer

There are two ways you could simplify your work  1  import Gson library  2  use Lombok   Both of them help you create String from object instance  Gson will parse your object  lombok will override your class object toString method   I put an example about Gson prettyPrint  I create helper class to print object and collection of objects  If you are using lombok  you could mark your class as  ToString and print your object directly    Scope value    prototype    Component public class DebugPrint lt T gt       public String PrettyPrint T obj         Gson gson   new GsonBuilder   setPrettyPrinting   create          return gson toJson obj           public String PrettyPrint Collection lt T gt  list         Gson gson   new GsonBuilder   setPrettyPrinting   create          return list stream   map gson  toJson  collect Collectors joining

User · Answer

You could convert it to an array and then print that out with Arrays toString Object      System out println Arrays toString stack toArray

User · Answer

If this is your own collection class rather than a built in one  you need to override its toString method  Eclipse calls that function for any objects for which it does not have a hard-wired formatting

User · Answer

Newer JDK already has AbstractCollection toString   implemented  and Stack extends AbstractCollection so you just have to call toString   on your collection      public abstract class AbstractCollection lt E gt  implements Collection lt E gt                public String toString             Iterator lt E gt  it   iterator            if    it hasNext                return  quot    quot                StringBuilder sb   new StringBuilder            sb append               for                    E e   it next                sb append e    this    quot  this Collection  quot    e               if    it hasNext                    return sb append      toString                sb append      append

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Be careful when calling Sop on Collection  it can throw ConcurrentModification Exception  Because internally toString method of each Collection internally calls Iterator over the Collection

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String join      yourIterable      Java 8

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Implement toString   on the class    I recommend the Apache Commons ToStringBuilder to make this easier  With it  you just have to write this sort of method   public String toString          return new ToStringBuilder this          append  name   name          append  age   age          toString         In order to get this sort of output       Person 7f54 name Stephen age 29    There is also a reflective implementation

User · Answer

System out println Collection c  already print any type of collection in readable format  Only if collection contains user defined objects   then you need to implement toString   in user defined class to display content

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JSON  An alternative Solution could be converting your collection in the JSON format and print the Json-String  The advantage is a well formatted and readable Object-String without a need of implementing the toString      Example using Google s Gson   import com google gson Gson  import com google gson GsonBuilder            printJsonString stack        public static void printJsonString Object o        GsonBuilder gsonBuilder   new GsonBuilder                  Some options for GsonBuilder like setting dateformat or pretty printing             Gson gson   gsonBuilder create        String json  gson toJson o       System out println json

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Should work for any collection except Map  but it s easy to support  too  Modify code to pass these 3 chars as arguments if needed   static  lt T gt  String seqToString Iterable lt T gt  items        StringBuilder sb   new StringBuilder        sb append           boolean needSeparator   false      for  T x   items            if  needSeparator              sb append               sb append x toString             needSeparator   true            sb append           return sb toString

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The MapUtils class offered by the Apache Commons project offers a MapUtils debugPrint method which will pretty print your map

User · Answer

You could convert it to an array and then print that out with Arrays toString Object      System out println Arrays toString stack toArray

User · Answer

I agree with the above comments about overriding toString   on your own classes  and about automating that process as much as possible    For classes you didn t define  you could write a ToStringHelper class with an overloaded method for each library class you want to have handled to your own tastes   public class ToStringHelper             instance configuration here  e g  punctuation  etc       public toString List m               presentation of List content to your liking           public toString Map m               presentation of Map content to your liking           public toString Set m               presentation of Set content to your liking                 etc      EDIT  Responding to the comment by xukxpvfzflbbld  here s a possible implementation for the cases mentioned previously   package com so demos   import java util List  import java util Map  import java util Set   public class ToStringHelper        private String separator      private String arrow       public ToStringHelper String separator  String arrow            this separator   separator          this arrow   arrow            public String toString List lt   gt  l            StringBuilder sb   new StringBuilder               String sep               for  Object object   l                sb append sep  append object toString                 sep   separator                    return sb append      toString               public String toString Map lt     gt  m            StringBuilder sb   new StringBuilder               String sep               for  Object object   m keySet                  sb append sep                 append object toString                   append arrow                 append m get object  toString                 sep   separator                    return sb append      toString               public String toString Set lt   gt  s            StringBuilder sb   new StringBuilder               String sep               for  Object object   s                sb append sep  append object toString                 sep   separator                    return sb append      toString               This isn t a full-blown implementation  but just a starter

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You can try using  org apache commons lang3 builder ToStringBuilder reflectionToString yourCollection

User · Answer

JSON  An alternative Solution could be converting your collection in the JSON format and print the Json-String  The advantage is a well formatted and readable Object-String without a need of implementing the toString      Example using Google s Gson   import com google gson Gson  import com google gson GsonBuilder            printJsonString stack        public static void printJsonString Object o        GsonBuilder gsonBuilder   new GsonBuilder                  Some options for GsonBuilder like setting dateformat or pretty printing             Gson gson   gsonBuilder create        String json  gson toJson o       System out println json

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I agree with the above comments about overriding toString   on your own classes  and about automating that process as much as possible    For classes you didn t define  you could write a ToStringHelper class with an overloaded method for each library class you want to have handled to your own tastes   public class ToStringHelper             instance configuration here  e g  punctuation  etc       public toString List m               presentation of List content to your liking           public toString Map m               presentation of Map content to your liking           public toString Set m               presentation of Set content to your liking                 etc      EDIT  Responding to the comment by xukxpvfzflbbld  here s a possible implementation for the cases mentioned previously   package com so demos   import java util List  import java util Map  import java util Set   public class ToStringHelper        private String separator      private String arrow       public ToStringHelper String separator  String arrow            this separator   separator          this arrow   arrow            public String toString List lt   gt  l            StringBuilder sb   new StringBuilder               String sep               for  Object object   l                sb append sep  append object toString                 sep   separator                    return sb append      toString               public String toString Map lt     gt  m            StringBuilder sb   new StringBuilder               String sep               for  Object object   m keySet                  sb append sep                 append object toString                   append arrow                 append m get object  toString                 sep   separator                    return sb append      toString               public String toString Set lt   gt  s            StringBuilder sb   new StringBuilder               String sep               for  Object object   s                sb append sep  append object toString                 sep   separator                    return sb append      toString               This isn t a full-blown implementation  but just a starter

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You can use the  Objects  class from JAVA  which is available since 1 7   Collection lt String gt  myCollection   Arrays asList  1273   123   876   897    Objects toString myCollection     Output  1273  123  876  897  Another possibility is to use the  MoreObjects  class from Google Guave  which provides many of useful helper functions   MoreObjects toStringHelper this  add  NameOfYourObject   myCollection  toString       Output      NameOfYourObject  1273  123  876  897   Guava docs

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Should work for any collection except Map  but it s easy to support  too  Modify code to pass these 3 chars as arguments if needed   static  lt T gt  String seqToString Iterable lt T gt  items        StringBuilder sb   new StringBuilder        sb append           boolean needSeparator   false      for  T x   items            if  needSeparator              sb append               sb append x toString             needSeparator   true            sb append           return sb toString

User · Answer

If this is your own collection class rather than a built in one  you need to override its toString method  Eclipse calls that function for any objects for which it does not have a hard-wired formatting

User · Answer

Implement toString   on the class    I recommend the Apache Commons ToStringBuilder to make this easier  With it  you just have to write this sort of method   public String toString          return new ToStringBuilder this          append  name   name          append  age   age          toString         In order to get this sort of output       Person 7f54 name Stephen age 29    There is also a reflective implementation

User · Answer

I agree with the above comments about overriding toString   on your own classes  and about automating that process as much as possible    For classes you didn t define  you could write a ToStringHelper class with an overloaded method for each library class you want to have handled to your own tastes   public class ToStringHelper             instance configuration here  e g  punctuation  etc       public toString List m               presentation of List content to your liking           public toString Map m               presentation of Map content to your liking           public toString Set m               presentation of Set content to your liking                 etc      EDIT  Responding to the comment by xukxpvfzflbbld  here s a possible implementation for the cases mentioned previously   package com so demos   import java util List  import java util Map  import java util Set   public class ToStringHelper        private String separator      private String arrow       public ToStringHelper String separator  String arrow            this separator   separator          this arrow   arrow            public String toString List lt   gt  l            StringBuilder sb   new StringBuilder               String sep               for  Object object   l                sb append sep  append object toString                 sep   separator                    return sb append      toString               public String toString Map lt     gt  m            StringBuilder sb   new StringBuilder               String sep               for  Object object   m keySet                  sb append sep                 append object toString                   append arrow                 append m get object  toString                 sep   separator                    return sb append      toString               public String toString Set lt   gt  s            StringBuilder sb   new StringBuilder               String sep               for  Object object   s                sb append sep  append object toString                 sep   separator                    return sb append      toString               This isn t a full-blown implementation  but just a starter

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Just Modified the previous example to print even collection containing user defined objects   public class ToStringHelper        private  static String separator     n        public ToStringHelper String seperator            super            ToStringHelper separator   seperator              public  static String toString List lt   gt  l            StringBuilder sb   new StringBuilder            String sep               for  Object object   l                String v   ToStringBuilder reflectionToString object               int start   v indexOf                   int end   v indexOf                   String st    v substring start end 1               sb append sep  append st               sep   separator                    return sb toString               public static String toString Map lt     gt  m            StringBuilder sb   new StringBuilder            String sep               for  Object object   m keySet                  String v   ToStringBuilder reflectionToString m get object                int start   v indexOf                   int end   v indexOf                   String st    v substring start end 1               sb append sep  append st               sep   separator                    return sb toString               public static String toString Set lt   gt  s            StringBuilder sb   new StringBuilder            String sep               for  Object object   s                String v   ToStringBuilder reflectionToString object               int start   v indexOf                   int end   v indexOf                   String st    v substring start end 1               sb append sep  append st               sep   separator                    return sb toString               public static void print List lt   gt  l            System out println toString l                  public static void print Map lt     gt  m            System out println toString m                  public static void print Set lt   gt  s            System out println toString s

User · Answer

With java 8 streams and collectors it can be done easily   String format Collection lt   gt  c      String s   c stream   map Object  toString  collect Collectors joining          return String format    s    s       first we use map with Object  toString to create Collection lt String gt  and then use joining collector to join every item in collection with   as delimiter

User · Answer

You could convert it to an array and then print that out with Arrays toString Object      System out println Arrays toString stack toArray

User · Answer

String join      yourIterable      Java 8

User · Answer

You can use the  Objects  class from JAVA  which is available since 1 7   Collection lt String gt  myCollection   Arrays asList  1273   123   876   897    Objects toString myCollection     Output  1273  123  876  897  Another possibility is to use the  MoreObjects  class from Google Guave  which provides many of useful helper functions   MoreObjects toStringHelper this  add  NameOfYourObject   myCollection  toString       Output      NameOfYourObject  1273  123  876  897   Guava docs

[java] Printing Java Collections Nicely (toString Doesn't Return Pretty Output)

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