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

Question

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.

标签： javaoop

Answer 1

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".

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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 :)

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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!