How to sort an array of objects in Java

Question

My array does not contain any string  But its contains object references  Every object reference returns name  id  author and publisher by toString method   public String toString             return  name     n    id     n    author     n    publisher     n        Now I need to sort that array of objects by the name  I know how to sort  but I do not know how to extract the name from the objects and sort them

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import java util Collections   import java util List   import java util ArrayList    public class Test    public static void main String   args        List lt Student gt  str   new ArrayList lt Student gt         str add new Student 101   aaa         str add new Student 104   bbb         str add new Student 103   ccc         str add new Student 105   ddd         str add new Student 104   eee         str add new Student 102   fff            Collections sort str       for Student student   str             System out println student

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Sometimes you want to sort an array of objects on an arbitrary value   Since compareTo   always uses the same information about the instance  you might want to use a different technique   One way is to use a standard sorting algorithm   Let s say you have an array of books and you want to sort them on their height  which is stored as an int and accessible through the method getHeight     Here s how you could sort the books in your array   If you don t want to change the original array  simply make a copy and sort that     int tallest     the index of tallest book found thus far  Book temp     used in the swap  for int a   0  a  lt  booksArray length - 1  a         tallest   a     reset tallest to current index       start inner loop at next index    for int b   a   1  b  lt  booksArray length  b            check if the book at this index is taller than the         tallest found thus far      if booksArray b  getHeight    gt  booksArray tallest  getHeight           tallest   b        once inner loop is complete  swap the tallest book found with       the one at the current index of the outer loop    temp   booksArray a      booksArray a    booksArray tallest      booksArray tallest    temp        When this code is done  the array of Book object will be sorted by height in descending order--an interior designer s dream

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You can try something like this   List lt Book gt  books   new ArrayList lt Book gt      Collections sort books  new Comparator lt Book gt        public int compare Book o1  Book o2           return o1 name compareTo o2 name

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Arrays sort yourList new Comparator lt YourObject gt            Override     public int compare YourObjecto1  YourObjecto2            return compare o1 getYourColumn    o2 getYourColumn

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Update for Java 8 constructs Assuming a Book class with a name field getter  you can use Arrays sort method by passing an additional Comparator specified using Java 8 constructs - Comparator default method  amp  method references  Arrays sort bookArray  Comparator comparing Book  getName     Also  it s possible to compare on multiple fields using thenComparing methods  Arrays sort bookArray  Comparator comparing Book  getName         thenComparing Book  getAuthor          thenComparingInt Book  getId

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With Java 8  you can use a reference method   You could add compare method to your Book class   class Book        public static int compare Book a  Book b                  return a name compareTo b name              And then you could do this    Arrays sort books   Book  compare     Here is the full example   class Book       String name      String author       public Book String name  String author            this name   name          this author   author             public static int compareBooks Book a   Book b                return a name compareTo b name               Override     public String toString             return  name       name     t     author       author              public static void main String   args             Book   books                     new Book  Book 3     Author 1                    new Book  Book 2     Author 2                    new Book  Book 1     Author 3                    new Book  Book 4     Author 4                       Arrays sort books   Book  compareBooks           Arrays asList books  forEach System out  println

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You have two ways to do that  both use the Arrays utility class   Implement a Comparator and pass your array along with the comparator to the sort method which take it as second parameter  Implement the Comparable interface in the class your objects are from and pass your array to the sort method which takes only one parameter    Example  class Book implements Comparable lt Book gt        public String name  id  author  publisher      public Book String name  String id  String author  String publisher            this name   name          this id   id          this author   author          this publisher   publisher            public String toString             return        name          id          author          publisher                    Override     public int compareTo Book o               usually toString should not be used             instead one of the attributes or more in a comparator chain         return toString   compareTo o toString               Test public void sortBooks         Book   books                 new Book  foo    1    author1    pub1                new Book  bar    2    author2    pub2                  1  sort using Comparable     Arrays sort books       System out println Arrays asList books            2  sort using comparator  sort by id     Arrays sort books  new Comparator lt Book gt               Override         public int compare Book o1  Book o2                return o1 id compareTo o2 id                         System out println Arrays asList books        Output    bar  2  author2  pub2    foo  1  author1  pub1     foo  1  author1  pub1    bar  2  author2  pub2

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Java 8    Using lambda expressions  Arrays sort myTypes   a b  - gt  a name compareTo b name      Test java  public class Test        public static void main String   args             MyType   myTypes                     new MyType  John   2   author1    publisher1                    new MyType  Marry   298   author2    publisher2                    new MyType  David   3   author3    publisher3                        System out println  --- before            System out println Arrays asList myTypes            Arrays sort myTypes   a  b  - gt  a name compareTo b name            System out println  --- after            System out println Arrays asList myTypes                MyType java  public class MyType        public String name      public int id      public String author      public String publisher       public MyType String name  int id  String author  String publisher            this name   name          this id   id          this author   author          this publisher   publisher              Override     public String toString             return  MyType                      name     name                             id     id                      author      author                             publisher      publisher                                System getProperty  line separator              Output   --- before  MyType name John   id 2  author  author1   publisher  publisher1     MyType name Marry   id 298  author  author2   publisher  publisher2     MyType name David   id 3  author  author3   publisher  publisher3     --- after  MyType name David   id 3  author  author3   publisher  publisher3     MyType name John   id 2  author  author1   publisher  publisher1     MyType name Marry   id 298  author  author2   publisher  publisher2         Using method references  Arrays sort myTypes  MyType  compareThem     where compareThem has to be added in MyType java   public static int compareThem MyType a  MyType b        return a name compareTo b name

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You can implement the  Comparable  interface on a class whose objects you want to compare   And also implement the  compareTo  method in that   Add the instances of the class in an ArrayList  Then the  java utils Collections sort    method will do the necessary magic   Here s---  https   deva-codes herokuapp com CompareOnTwoKeys  a working example where objects are sorted based on two keys first by the id and then by name

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Employee name John  age 25  salary 3000 0  mobile 9922001    Employee name Ace  age 22  salary 2000 0  mobile 5924001    Employee name Keith  age 35  salary 4000 0  mobile 3924401     public void whenComparing thenSortedByName     Comparator lt Employee gt  employeeNameComparator     Comparator comparing Employee  getName    Arrays sort employees  employeeNameComparator    assertTrue Arrays equals employees  sortedEmployeesByName       result  Employee name Ace  age 22  salary 2000 0  mobile 5924001    Employee name John  age 25  salary 3000 0  mobile 9922001    Employee name Keith  age 35  salary 4000 0  mobile 3924401

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public class Student implements Comparable lt Student gt         private int sid      private String sname       public Student int sid  String sname            super            this sid   sid          this sname   sname             public int getSid             return sid             public void setSid int sid            this sid   sid             public String getSname             return sname             public void setSname String sname            this sname   sname              Override     public String toString             return  Student  sid     sid      sname     sname                   public int compareTo Student o            if  this getSname   compareTo o getSname     gt  1                return toString   compareTo o getSname               else if  this getSname   compareTo o getSname     lt  1                return toString   compareTo o getSname                       return 0

[java] How to sort an array of objects in Java?

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