LambdaCalculator.java - Lambda Expression Example

Java Tutorials - Herong's Tutorial Examples

∟Lambda Expressions and Method References

∟LambdaCalculator.java - Lambda Expression Example

This section provides a tutorial example on using an lambda expression to define a class anonymously implementing a single abstract method interface. An anonymous class and a local class are used as comparisons.

My first lambda expression program, LambdaCalculator.java, follows an example given in a Oracle Java tutorial. Here is the source code:

/* LambdaCalculator.java
 - Copyright (c) 2014, HerongYang.com, All Rights Reserved.
 */
class LambdaCalculator {
   interface Operation {
      int operate(int a, int b);   
   }
   public int calculate(int a, int b, Operation o) {
      System.out.println(o.toString());
      return o.operate(a, b);
   }

   public static void main(String[] arg) {
      int x = 1;
      String s = "+";
      int y = 2;
      if (arg.length>0) x = Integer.parseInt(arg[0]);
      if (arg.length>1) s = arg[1];
      if (arg.length>2) y = Integer.parseInt(arg[2]);

      // Using a lambda expression
      Operation add = (int a, int b) -> {return a+b;};
      
      // Using anonymous class
      Operation subtract = new Operation() { 
      	 public int operate(int a, int b) {return a-b;} 
      };
      
      // Using a local class
      class Multiply implements Operation {
      	 public int operate(int a, int b) {return a*b;} 
      }
      Operation multiply = new Multiply();

      LambdaCalculator c = new LambdaCalculator();
      if (s.equals("+")) 
         System.out.println(x+s+y+" = "+c.calculate(x,y,add));
      else if (s.equals("-")) 
         System.out.println(x+s+y+" = "+c.calculate(x,y,subtract));
      else if (s.equals("*")) 
         System.out.println(x+s+y+" = "+c.calculate(x,y,multiply));
      else
         System.out.println(x+s+y+" = Undefined");
   }
}

Note that my sample program uses an lambda expression, an anonymous class and a local class to implement the same interface:

"Operation" is a single abstract method interface which is nested in the "LambdaCalculator" class.
"Operation add = (int a, int b) -> {return a+b;};" uses a lambda expression to implement an add operation. A new object is created from the implementation class and assigned to "add".
"Operation subtract = new Operation() { ... };" uses an anonymous class to implement a subtract operation. A new object is created from the implementation class and assigned to "subtract".
"Operation multiply = new Multiply();" uses a local class to implement a multiply operation. A new object is created from the implementation class and assigned to "multiply".

Now compile and run LambdaCalculator.java with JDK 1.8, you will get:

C:\herong>java LambdaCalculator
LambdaCalculator$$Lambda$1/518248@cf9800
1+2 = 3

C:\herong>java LambdaCalculator 8 + 2
LambdaCalculator$$Lambda$1/518248@cf9800
8+2 = 10

C:\herong>java LambdaCalculator 8 - 2
LambdaCalculator$1@cf9800
8-2 = 6

C:\herong>java LambdaCalculator 8 "*" 2
LambdaCalculator$1Multiply@cf9800
8*2 = 16

Here is what I learned from the output:

The lambda expression "(a,b) -> {return a+b;}" does define a class named as "LambdaCalculator$$Lambda$1/518248".
When using * as command line argument, it needs to be quoted as "*", because * is wildcard character on Windows systems.

If we look at class files generated by the compiler, we will see only 4 of them. Any there is no class file for the "LambdaCalculator$$Lambda$1/518248" class defined by the lambda expression.

C:\herong>dir LambdaCalculator*.*

1,423 LambdaCalculator.java
2,076 LambdaCalculator.class           - Main application class 
  214 LambdaCalculator$Operation.class - Nested interface "Operation"
  440 LambdaCalculator$1.class         - Anonymous class for "subtract
  459 LambdaCalculator$1Multiply.class - Local class "Multiply"

Last update: 2014.

Table of Contents

About This Book

Installing JDK 1.8 on Windows

Execution Process, Entry Point, Input and Output

Primitive Data Types and Literals

Bits, Bytes, Bitwise and Shift Operations

Managing Bit Strings in Byte Arrays

Reference Data Types and Variables

StringBuffer - The String Buffer Class

System Properties and Runtime Object Methods

Generic Classes and Parameterized Types