What causes javac to issue the uses unchecked or unsafe operations warning

Question

For example   javac Foo java Note  Foo java uses unchecked or unsafe operations  Note  Recompile with -Xlint unchecked for details

User · Accepted Answer

This comes up in Java 5 and later if you're using collections without type specifiers (e.g., Arraylist() instead of ArrayList<String>()). It means that the compiler can't check that you're using the collection in a type-safe way, using generics.

To get rid of the warning, just be specific about what type of objects you're storing in the collection. So, instead of

List myList = new ArrayList();

use

List<String> myList = new ArrayList<String>();

In Java 7 you can shorten generic instantiation by using Type Inference.

List<String> myList = new ArrayList<>();

User · Answer

If you do what it suggests and recompile with the  -Xlint unchecked  switch  it will give you more detailed information   As well as the use of raw types  as described by the other answers   an unchecked cast can also cause the warning   Once you ve compiled with -Xlint  you should be able to rework your code to avoid the warning   This is not always possible  particularly if you are integrating with legacy code that cannot be changed   In this situation  you may decide to suppress the warning in places where you know that the code is correct    SuppressWarnings  unchecked   public void myMethod

User · Answer

I had 2 years old classes and some new classes  I solved it in Android Studio as follows   allprojects        gradle projectsEvaluated           tasks withType JavaCompile                options compilerArgs  lt  lt   -Xlint unchecked                       In my project build gradle file  Borzh solution   And then if some Metheds is left    SuppressWarnings  unchecked   public void myMethod

User · Answer

You can keep it in the generic form and write it as      list 2 is made generic and can store any type of Object         ArrayList lt Object gt  list2   new ArrayList lt Object gt       Setting type of ArrayList as Object gives us the advantage to store any type of data  You don t need to use -Xlint or anything else

User · Answer

The  unchecked or unsafe operations  warning was added when java added Generics  if I remember correctly  It s usually asking you to be more explicit about types  in one way or another    For example  the code ArrayList foo   new ArrayList    triggers that warning because javac is looking for ArrayList lt String gt  foo   new ArrayList lt String gt

User · Answer

This warning means that your code operates on a raw type  recompile the example with the   -Xlint unchecked    to get the details   like this     javac YourFile java -Xlint unchecked  Main java 7  warning   unchecked  unchecked cast         clone mylist    ArrayList lt String gt  this mylist clone                                                                   required  ArrayList lt String gt    found     Object 1 warning   docs oracle com talks about it here   http   docs oracle com javase tutorial java generics rawTypes html

User · Answer

For Android Studio  you need to add   allprojects        gradle projectsEvaluated           tasks withType JavaCompile                options compilerArgs  lt  lt   -Xlint unchecked                                  in your project s build gradle file to know where this error is produced

User · Answer

for example when you call a function that returns Generic Collections and you don t specify the generic parameters yourself   for a function  List lt String gt  getNames     List names   obj getNames      will generate this error   To solve it you would just add the parameters  List lt String gt  names   obj getNames

User · Answer

I had 2 years old classes and some new classes  I solved it in Android Studio as follows   allprojects        gradle projectsEvaluated           tasks withType JavaCompile                options compilerArgs  lt  lt   -Xlint unchecked                       In my project build gradle file  Borzh solution   And then if some Metheds is left    SuppressWarnings  unchecked   public void myMethod

User · Answer

The  unchecked or unsafe operations  warning was added when java added Generics  if I remember correctly  It s usually asking you to be more explicit about types  in one way or another    For example  the code ArrayList foo   new ArrayList    triggers that warning because javac is looking for ArrayList lt String gt  foo   new ArrayList lt String gt

User · Answer

The  unchecked or unsafe operations  warning was added when java added Generics  if I remember correctly  It s usually asking you to be more explicit about types  in one way or another    For example  the code ArrayList foo   new ArrayList    triggers that warning because javac is looking for ArrayList lt String gt  foo   new ArrayList lt String gt

User · Answer

If you do what it suggests and recompile with the  -Xlint unchecked  switch  it will give you more detailed information   As well as the use of raw types  as described by the other answers   an unchecked cast can also cause the warning   Once you ve compiled with -Xlint  you should be able to rework your code to avoid the warning   This is not always possible  particularly if you are integrating with legacy code that cannot be changed   In this situation  you may decide to suppress the warning in places where you know that the code is correct    SuppressWarnings  unchecked   public void myMethod

User · Answer

For Android Studio  you need to add   allprojects        gradle projectsEvaluated           tasks withType JavaCompile                options compilerArgs  lt  lt   -Xlint unchecked                                  in your project s build gradle file to know where this error is produced

User · Answer

This warning means that your code operates on a raw type  recompile the example with the   -Xlint unchecked    to get the details   like this     javac YourFile java -Xlint unchecked  Main java 7  warning   unchecked  unchecked cast         clone mylist    ArrayList lt String gt  this mylist clone                                                                   required  ArrayList lt String gt    found     Object 1 warning   docs oracle com talks about it here   http   docs oracle com javase tutorial java generics rawTypes html

User · Answer

This warning also could be raised due to   new HashMap    or new ArrayList   that is generic type has to be specific otherwise the compiler will generate warning   Please make sure that if you code contains the following you have to change accordingly  new HashMap      Map map   new HashMap   new HashMap      Map map   new HashMap lt      new ArrayList      List map   new ArrayList   new ArrayList      List map   new ArrayList lt

User · Answer

The solution would be to use specific type in  lt  gt  like ArrayList lt File gt    example    File curfolder   new File   C   Users  username  Desktop    File   file   curfolder listFiles    ArrayList filename   Arrays asList file     above code generate warning because ArrayList is not of specific type   File curfolder   new File   C   Users  username  Desktop    File   file   curfolder listFiles    ArrayList lt File gt  filename   Arrays asList file     above code will do fine  Only change is in third line after ArrayList

User · Answer

This warning also could be raised due to   new HashMap    or new ArrayList   that is generic type has to be specific otherwise the compiler will generate warning   Please make sure that if you code contains the following you have to change accordingly  new HashMap      Map map   new HashMap   new HashMap      Map map   new HashMap lt      new ArrayList      List map   new ArrayList   new ArrayList      List map   new ArrayList lt

User · Answer

I have ArrayList lt Map lt String  Object gt  gt  items    ArrayList lt Map lt String  Object gt  gt   value   Because value is a complex structure  I want to clean JSON   there can happen any combinations on numbers  booleans  strings  arrays  So  I used the solution of  Dan Dyer    SuppressWarnings  unchecked   ArrayList lt Map lt String  Object gt  gt  items    ArrayList lt Map lt String  Object gt  gt   value

User · Answer

The solution would be to use specific type in  lt  gt  like ArrayList lt File gt    example    File curfolder   new File   C   Users  username  Desktop    File   file   curfolder listFiles    ArrayList filename   Arrays asList file     above code generate warning because ArrayList is not of specific type   File curfolder   new File   C   Users  username  Desktop    File   file   curfolder listFiles    ArrayList lt File gt  filename   Arrays asList file     above code will do fine  Only change is in third line after ArrayList

User · Answer

I just want to add one example of the kind of unchecked warning I see quite often  If you use classes that implement an interface like Serializable  often you will call methods that return objects of the interface  and not the actual class  If the class being returned must be cast to a type based on generics  you can get this warning    Here is a brief  and somewhat silly  example to demonstrate   import java io Serializable   public class SimpleGenericClass lt T gt  implements Serializable        public Serializable getInstance             return this                 SuppressWarnings  unchecked       public static void main              SimpleGenericClass lt String gt  original   new SimpleGenericClass lt String gt                  java  unchecked cast               required  SimpleGenericClass lt java lang String gt                found     java io Serializable         SimpleGenericClass lt String gt  returned                    SimpleGenericClass lt String gt   original getInstance              getInstance   returns an object that implements Serializable  This must be cast to the actual type  but this is an unchecked cast

User · Answer

I have ArrayList lt Map lt String  Object gt  gt  items    ArrayList lt Map lt String  Object gt  gt   value   Because value is a complex structure  I want to clean JSON   there can happen any combinations on numbers  booleans  strings  arrays  So  I used the solution of  Dan Dyer    SuppressWarnings  unchecked   ArrayList lt Map lt String  Object gt  gt  items    ArrayList lt Map lt String  Object gt  gt   value

User · Answer

You can keep it in the generic form and write it as      list 2 is made generic and can store any type of Object         ArrayList lt Object gt  list2   new ArrayList lt Object gt       Setting type of ArrayList as Object gives us the advantage to store any type of data  You don t need to use -Xlint or anything else

User · Answer

The  unchecked or unsafe operations  warning was added when java added Generics  if I remember correctly  It s usually asking you to be more explicit about types  in one way or another    For example  the code ArrayList foo   new ArrayList    triggers that warning because javac is looking for ArrayList lt String gt  foo   new ArrayList lt String gt

User · Answer

If you do what it suggests and recompile with the  -Xlint unchecked  switch  it will give you more detailed information   As well as the use of raw types  as described by the other answers   an unchecked cast can also cause the warning   Once you ve compiled with -Xlint  you should be able to rework your code to avoid the warning   This is not always possible  particularly if you are integrating with legacy code that cannot be changed   In this situation  you may decide to suppress the warning in places where you know that the code is correct    SuppressWarnings  unchecked   public void myMethod

User · Answer

for example when you call a function that returns Generic Collections and you don t specify the generic parameters yourself   for a function  List lt String gt  getNames     List names   obj getNames      will generate this error   To solve it you would just add the parameters  List lt String gt  names   obj getNames

User · Answer

I just want to add one example of the kind of unchecked warning I see quite often  If you use classes that implement an interface like Serializable  often you will call methods that return objects of the interface  and not the actual class  If the class being returned must be cast to a type based on generics  you can get this warning    Here is a brief  and somewhat silly  example to demonstrate   import java io Serializable   public class SimpleGenericClass lt T gt  implements Serializable        public Serializable getInstance             return this                 SuppressWarnings  unchecked       public static void main              SimpleGenericClass lt String gt  original   new SimpleGenericClass lt String gt                  java  unchecked cast               required  SimpleGenericClass lt java lang String gt                found     java io Serializable         SimpleGenericClass lt String gt  returned                    SimpleGenericClass lt String gt   original getInstance              getInstance   returns an object that implements Serializable  This must be cast to the actual type  but this is an unchecked cast

User · Answer

for example when you call a function that returns Generic Collections and you don t specify the generic parameters yourself   for a function  List lt String gt  getNames     List names   obj getNames      will generate this error   To solve it you would just add the parameters  List lt String gt  names   obj getNames

User · Answer

If you do what it suggests and recompile with the  -Xlint unchecked  switch  it will give you more detailed information   As well as the use of raw types  as described by the other answers   an unchecked cast can also cause the warning   Once you ve compiled with -Xlint  you should be able to rework your code to avoid the warning   This is not always possible  particularly if you are integrating with legacy code that cannot be changed   In this situation  you may decide to suppress the warning in places where you know that the code is correct    SuppressWarnings  unchecked   public void myMethod

User · Answer

for example when you call a function that returns Generic Collections and you don t specify the generic parameters yourself   for a function  List lt String gt  getNames     List names   obj getNames      will generate this error   To solve it you would just add the parameters  List lt String gt  names   obj getNames

[java] What causes javac to issue the "uses unchecked or unsafe operations" warning

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