Anonymous inner classes vs Lambda Expressions :

1) class Test { 
2) public static void main(String[] args) { 
3) Thread t = new Thread(new Runnable() { 
4) public void run() { 
5) for(int i=0; i<10; i++) { 
6) System.out.println("Child Thread"); 
7) } 
8) } 
9) }); 
10) t.start(); 
11) for(int i=0; i<10; i++) 
12) System.out.println("Main thread"); 
13) } 
14) }

1) class Test { 
2) public static void main(String[] args) { 
3) Thread t = new Thread(() -> { 
4) for(int i=0; i<10; i++) { 
5) System.out.println("Child Thread"); 
6) } 
7) }); 
8) t.start(); 
9) for(int i=0; i<10; i++) { 
10) System.out.println("Main Thread"); 
11) } 
12) } 
13) }

• We can reduce length of the code so that readability of the code will be improved.
• We can resolve complexity of anonymous inner classes.
• We can provide Lambda expression in the place of object.
• We can pass lambda expression as argument to methods.

• Anonymous inner class can extend concrete class, can extend abstract class, can implement interface with any number of methods but Lambda expression can implement an interface with only single abstract method (Functional Interface).
• Hence if anonymous inner class implements Functional Interface in that particular case only we can replace with lambda expressions. Hence wherever anonymous inner class concept is there, it may not possible to replace with Lambda expressions.
• Anonymous inner class! = Lambda Expression
• Inside anonymous inner class we can declare instance variables.
• Inside anonymous inner class "this" always refers current inner class object(anonymous inner class) but not related outer class object

• Inside lambda expression we can't declare instance variables.
• Whatever the variables declare inside lambda expression are simply acts as local variables
• Within lambda expression "this" keyword represents current outer class object reference (that is current enclosing class reference in which we declare lambda expression)

Ex: 
1) interface Interf { 
2) public void m1(); 
3) } 
4) class Test { 
5) int x = 777; 
6) public void m2() { 
7) Interf i = ()-> { 
8) int x = 888; 
9) System.out.println(x); //888 
10) System.out.println(this.x); //777 
11) }; 
12) i.m1(); 
13) } 
14) public static void main(String[] args) { 
15) Test t = new Test(); 
16) t.m2(); 
17) } 
18) }

• From lambda expression we can access enclosing class variables and enclosing method variables directly.
• The local variables referenced from lambda expression are implicitly final and hence we can't perform re-assignment for those local variables otherwise we get compile time error

Ex: 
1) interface Interf { 
2) public void m1(); 
3) } 
4) class Test { 
5) int x = 10; 
6) public void m2() { 
7) int y = 20; 
8) Interf i = () -> { 
9) System.out.println(x); //10 
10) System.out.println(y); //20 
11) x = 888; 
12) y = 999; //CE 
13) }; 
14) i.m1(); 
15) y = 777; 
16) } 
17) public static void main(String[] args) { 
18) Test t = new Test(); 
19) t.m2();
20) } 
21) }