Method calls, the details -

Figure 343. Method calling

public class Circle {
  static final double
    PI = 3.141592653589793;
  double r;
  /** Change a circle's area
   *  @param area The desired new area
   *  @return The circle's new radius */
  double setArea(final double area ❶) {
    double val ❷ = area  / PI ❸;
    return ❹ r ❺ = Math.sqrt(val);
  }
}

❶	Passing arguments.
❷	Defining method local variables.
❸	Accessing class variable.

❹	returning values.

❺	Accessing instance variable.

Figure 344. Three variable scopes

public class Circle {

  static final double
    PI ❶ = 3.141592653589793;

  double r ❷ ;

  double setArea(final double area ❸) {
    double val ❸ ...
    ...
  }
}

❶	Class scope.
❷	Instance scope
❸	Method scope

Figure 345. Scope lifetimes

Class scope (`static`)	Application process
Instance scope	Object lifetime: `new (...)` until being garbage collected.
Method scope	Method invocation until `return`.

Figure 346. Two runtime memory categories

Heap memory

Allocation of class or array instances:

new String()

new float[200]

De-allocation subject to garbage collection.

Execution stack

One instance per process thread.
Hosting variables (values or references)

Figure 347. Stack: Four operations

Method	Description	Precondition	Access type
`push(...)`	Add object on top	-	Modify
`pop()`	Read + remove topmost object	Stack not empty	Modify
`top()`	Read topmost object	Stack not empty	Read only
`empty()`	`true` if and only if stack is empty	-	Read only

The JDK™ implements a peek() method having exactly the «original» stack top() method's semantic.

Figure 348. Example: Storing integer values

‹›

Figure 349. Method calling

‹›

Figure 350. Call stack trace

‹›

Figure 351. IDE debugger

❶	Three call stack frames corresponding to `main()` calling `circleArea(2)` calling `square(2)`.
❷	Local variables corresponding to selected stack frame. Tip In the above example you may select stack frame “circleArea” showing its local variables while leaving your debugger resting at line 3.

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Method calls, the details

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