How to declare a method that cannot change the class members in Java?

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In C++ we do a constant method, and so it can't change the value of the class members, but how can I do it in Java? I tried the final method declaration, which would be the equivalent to the consts in C++, but final methods are another different thing. I want to make a getter method, so it cannot change the values, only read it. Like a read-only method.

oop java
2 Answers

In Java, it's not possible to declaratively prevent a method from changing non-final fields. There's no such thing as "const correctness" in Java.

If a class field is non-final, then it can be changed by any method of the class.

Note that final works differently on fields and variables versus methods and classes:

  • A final field or variable is a constant. Its value cannot be changed once assigned.
  • A final method cannot be overridden by child classes. final on methods has nothing to do with "constness".
  • A final class cannot be extended. final on classes has nothing to do with "constness".

Your options in Java are:

  1. Make your class immutable (i.e. mark all its fields final).
  2. Just write your getters (and other non-mutator methods) carefully :)

One more note. Even on fields and variables, Java's final differs from C++'s const. For example:

  • final variables can't be changed once assigned, but you can assign them after the declaration. Example:

      final int value;
  if (condition) {
      value = 1; // Ok!
  } else {
      value = 2; // Ok!
  }
  value = 3; // Compile error: value already assigned.

  • final class fields must be assigned - either directly or from within a constructor. But these fields may be assigned anywhere within a constructor. There's no special thing like C++'s "initializer list" in Java.

  • The field/variable/parameter declaration final Object obj in Java is roughly equivalent to a (const-)reference to a non-const object in C++. This means, you can't re-assign the reference, but you can change the referenced object itself. An example:

      // Note that (obsolete) Date class is mutable in Java.
  final Date myDate = new Date();

  myDate = new Date(); // Compilation error: can't reassign a final reference!

  myDate.setTime(4711); // Ok, mutating referenced object is allowed!
user_21701920

You can not do that. Final method means it can't be overriden.

user_135394620
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