Why Does the State of a Java Object Remain Unchanged After a Method Call?

What causes a Java object’s state to remain unchanged after invoking a method?

public class MyClass {
    private int value;

    public MyClass(int value) {
        this.value = value;
    }

    public void changeValue(int newValue) {
        value = newValue;
    }

    public int getValue() {
        return value;
    }
}
// Usage:
MyClass obj = new MyClass(10);
obj.changeValue(20); // This changes the state of 'obj'.

In Java, when the state of an object does not change after invoking a method, it can often be attributed to how parameters are passed or how the method itself is implemented. This documented behavior is crucial for developers to understand in order to effectively manipulate object states.

// Example demonstrating unchanged object state:
public class Test {
    public static void main(String[] args) {
        MyClass obj = new MyClass(10);
        // Method that does not change the state:
        obj.changeValue(20);
        System.out.println(obj.getValue()); // Prints 20
    }
}

Causes

• The method does not alter the instance variables of the object.
• You're using a local copy of an object rather than the original one.
• Method parameters passed by value, causing the original object to remain unchanged.
• Calling a method that is not intended to change the state of the object.

Solutions

• Ensure the method modifies instance variables correctly.
• Use method chaining to return the modified object when needed.
• Check parameter types and make sure you're passing the correct object reference.
• Verify method access modifiers—ensure the method has access to the object’s state.

Mistake: Assuming object references do not have to be passed carefully in calls.

Solution: Always double-check if you're passing the correct reference and the method is designed to modify the intended object.

Mistake: Not returning the object within method chaining.

Solution: Use return statements in methods designed to effectuate changes such that the modified object can be used subsequently.