Java Nested Classes – java inner class, static nested class, local inner class and anonymous inner class

Java nested classes are defined as class inside the body of another class. A nested class can be declared private, public, protected, or with default access whereas an outer class can have only public or default access.

Nested classes are further divided into two types:

  1. static nested classes: If the nested class is static, then it’s called static nested class. Static nested classes can access only static members of the outer class. Static nested class is same as any other top-level class and is nested for only packaging convenience.

    Static class object can be created with following statement:

    OuterClass.StaticNestedClass nestedObject =
         new OuterClass.StaticNestedClass();
    
  2. java inner class: Any non-static nested class is known as inner class. Inner classes are associated with the object of the class and they can access all the variables and methods of the outer class. Since inner classes are associated with instance, we can’t have any static variables in them. Object of inner class are part of the outer class object and to create an instance of inner class, we first need to create instance of outer class.

    Inner classes can be instantiated like this:

    OuterClass outerObject = new OuterClass();
    OuterClass.InnerClass innerObject = outerObject.new InnerClass();
    

There are two special kinds of java inner classes.

  1. local inner class: If a class is defined in a method body, it’s known as local inner class. Since local inner class is not associated with Object, we can’t use private, public or protected access modifiers with it. The only allowed modifiers are abstract or final. A local inner class can access all the members of the enclosing class and local final variables in the scope it’s defined.

    Local inner class can be defined as:

    public void print() {
            //local inner class inside the method
            class Logger {
                String name;
            }
            //instantiate local inner class in the method to use
            Logger logger = new Logger();
    
  2. anonymous inner class: A local inner class without name is known as anonymous inner class. An anonymous class is defined and instantiated in a single statement. Anonymous inner class always extend a class or implement an interface. Since an anonymous class has no name, it is not possible to define a constructor for an anonymous class. Anonymous inner classes are accessible only at the point where it is defined.
    It’s a bit hard to define how to create anonymous inner class, we will see it’s real time usage in test program below.

Here is a java class showing how to define java inner class, static nested class, local inner class and anonymous inner class.

package com.journaldev.nested;


import java.io.File;
import java.io.FilenameFilter;


public class OuterClass {
    
    private static String name = "OuterClass";
    private int i;
    protected int j;
    int k;
    public int l;

    //OuterClass constructor
    public OuterClass(int i, int j, int k, int l) {
        this.i = i;
        this.j = j;
        this.k = k;
        this.l = l;
    }


    public int getI() {
        return this.i;
    }


    //static nested class, can access OuterClass static variables/methods
    static class StaticNestedClass {
        private int a;
        protected int b;
        int c;
        public int d;


        public int getA() {
            return this.a;
        }


        public String getName() {
            return name;
        }
    }


    //inner class, non static and can access all the variables/methods of outer class
    class InnerClass {
        private int w;
        protected int x;
        int y;
        public int z;


        public int getW() {
            return this.w;
        }


        public void setValues() {
            this.w = i;
            this.x = j;
            this.y = k;
            this.z = l;
        }


        @Override
        public String toString() {
            return "w=" + w + ":x=" + x + ":y=" + y + ":z=" + z;
        }


        public String getName() {
            return name;
        }
    }


    //local inner class
    public void print(String initial) {
        //local inner class inside the method
        class Logger {
            String name;


            public Logger(String name) {
                this.name = name;
            }


            public void log(String str) {
                System.out.println(this.name + ": " + str);
            }
        }

        Logger logger = new Logger(initial);
        logger.log(name);
        logger.log("" + this.i);
        logger.log("" + this.j);
        logger.log("" + this.k);
        logger.log("" + this.l);
    }


    //anonymous inner class
    public String[] getFilesInDir(String dir, final String ext) {
        File file = new File(dir);
        //anonymous inner class implementing FilenameFilter interface
        String[] filesList = file.list(new FilenameFilter() {

            @Override
            public boolean accept(File dir, String name) {
                return name.endsWith(ext);
            }

        });
        return filesList;
    }
}

Here is the test program showing how to instantiate and use nested class in java.

package com.journaldev.nested;

import java.util.Arrays;
//nested classes can be used in import for easy instantiation
import com.journaldev.nested.OuterClass.InnerClass;
import com.journaldev.nested.OuterClass.StaticNestedClass;

public class NestedClassTest {

    public static void main(String[] args) {
        OuterClass outer = new OuterClass(1,2,3,4);
        
        //static nested classes example
        StaticNestedClass staticNestedClass = new StaticNestedClass();
        StaticNestedClass staticNestedClass1 = new StaticNestedClass();
        
        System.out.println(staticNestedClass.getName());
        staticNestedClass.d=10;
        System.out.println(staticNestedClass.d);
        System.out.println(staticNestedClass1.d);
        
        //inner class example
        InnerClass innerClass = outer.new InnerClass();
        System.out.println(innerClass.getName());
        System.out.println(innerClass);
        innerClass.setValues();
        System.out.println(innerClass);
        
        //calling method using local inner class
        outer.print("Outer");
        
        //calling method using anonymous inner class
        System.out.println(Arrays.toString(outer.getFilesInDir("src/com/journaldev/nested", ".java")));
        
        System.out.println(Arrays.toString(outer.getFilesInDir("bin/com/journaldev/nested", ".class")));
    }

}

Here is the output of above program:

OuterClass
10
0
OuterClass
w=0:x=0:y=0:z=0
w=1:x=2:y=3:z=4
Outer: OuterClass
Outer: 1
Outer: 2
Outer: 3
Outer: 4
[NestedClassTest.java, OuterClass.java]
[NestedClassTest.class, OuterClass$1.class, OuterClass$1Logger.class, OuterClass$InnerClass.class, OuterClass$StaticNestedClass.class, OuterClass.class]

Notice that when OuterClass is compiled, separate class files are created for inner class, local inner class and static nested class.

Benefits of Java Nested Class

  1. If a class is useful to only one class, it makes sense to keep it nested and together. It helps in packaging of the classes.
  2. Nested classes increases encapsulation. Note that inner classes can access outer class private members and at the same time we can hide inner class from outer world.
  3. Nesting small classes within top-level classes places the code closer to where it is used and makes code more readable and maintainable.

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