15. Local variables

Local variables are local to methods or the code block in which they are defined. Parameters that are passed into a method as value, and not as reference such as arrays, are also local. For example, the primitives will be copied and passed as value.

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Instance and class variables must be defined outside any method. Also if two methods need access to one variable, it has to be instance or class variable.

 // *** 1. Start (Instance and Class)
 int a = 1; // Instance
 static int b = 2; // Class
 // *** 1. End

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Local variables lose scope when the block in which are defined ends with }. Should we want to save it, we have to return it. We can only return one value. But if an array is passed into a function as a parameter, it is not local but references the actual address and thus any changes will persist, regardless of what is returned.

  // *** 2. Start (main() method)
  int b = 2; // Local to main()
  System.out.println(b); // local
  System.out.println(Ex15.b); // static
  Ex15 obj1 = new Ex15();
  System.out.println(obj1.a); // instance
  obj1.ex15(); // call ex15()
  // *** 2. End

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The this keywords is used to access instance variables. It is especially useful if a local variable exists with the same name.

  // *** 3. Start (ex15() method)
  int b = 3; // Local to ex15()
  int a = 4; // Local to ex15()
  System.out.println(b); // local
  System.out.println((new Ex15()).a); // instance
  System.out.println(a); // local
  System.out.println(this.a); // instance
  System.out.println(this.b); // warning
  System.out.println(Ex15.b); // class ( correct way)
  // *** 3. End 

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// Ex15.java
package com.javaAndroid.ex15;

public class Ex15 {
 // *** 1. Start (Instance and Class)
 int a = 1; // Instance
 static int b = 2; // Class
 // *** 1. End
 public static void main(String[] args) {
  // *** 2. Start (main() method)
  int b = 2; // Local to main()
  System.out.println(b); // local
  System.out.println(Ex15.b); // static
  Ex15 obj1 = new Ex15();
  System.out.println(obj1.a); // instance
  obj1.ex15(); // call ex15()
  // *** 2. End
 }
 // 
 void ex15() {
  // *** 3. Start (ex15() method)
  int b = 3; // Local to ex15()
  int a = 4; // Local to ex15()
  System.out.println(b); // local
  System.out.println((new Ex15()).a); // instance
  System.out.println(a); // local
  System.out.println(this.a); // instance
  System.out.println(this.b); // warning
  System.out.println(Ex15.b); // class ( correct way)
  // *** 3. End 
 }
}

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