Operators and Assignments - Method Invocation

• when you pass a primitive value to a method, a copy of the value is made available to the method, not the value itself
• any changes made to the value in the method do not affect the original value

    int i = 50;
    changeValue(i);     // where method multiplies i by 3
    
Output:
    Original value of i:                     -> 50
    Value of i in the method:                -> 150
    Value of i after method invocation:      -> 50
  

Passing object references

• when you pass an object reference to a method, a copy of the reference is passed. Operations in the method which change the object reference do not affect the original; however, changes to the object itself within the method affect the original object

int[] array = { 10,10,10 }   // original array
changeObjectReference(array) // set the reference to a new array
changeActualObject(array)    // set the 2nd element of the array
    
Output:
  Original array values:                      10, 10, 10
  Array in the method:                        20, 20, 20
  After Object reference changed in method:   10, 10, 10
  After object changed in method:             10, 20, 10    

Method invocation conversion (JLS §5.3)

• each argument is converted to the type of the method parameters
• widening conversion is implicit
• narrowing conversion is not implicit (values must be cast)

Also see:

• Conversion
• Understanding that parameters are passed by value and not by reference

Example Code

• TestMethodInvocation.java

Traps

• code that results in a primitive value being changed in a method (can't happen)
• code that results in an unchanged object value when it was changed in a method
• failing to cast a value to match a method parameter type ie assuming narrowing conversion on a method call