Interfaces

Here is some code from the lesson on strings.

String1.java
1	public class String1 {
2	static void printChars(String s) {
3	System.out.println("String: "+s);
4	for(int i=0;i<s.length();i++) {
5	System.out.println("["+i+"] "+s.charAt(i));
6	}
7	}
8	public static void main(String[] args) {
9	for(int i=0;i<args.length;i++) {
10	printChars(args[i]);
11	}
12	}
13	}

We created a method called printChars() that listed all the characters in a String. Later we introduced the StringBuffer, which was very similar to the String but was mutable. Is it possible to modify this method to work with either String or StringBuffer? The answer is yes:

Interf1.java
1	public class Interf1 {
2	static void printChars(CharSequence s) {
3	System.out.println("CharSeq: "+s);
4	for(int i=0;i<s.length();i++) {
5	System.out.println("["+i+"] "+s.charAt(i));
6	}
7	}
8	public static void main(String[] args) {
9	String s = "Hello";
10	StringBuffer sb = new StringBuffer();
11	sb.append("Wor");
12	sb.append("ld");
13	printChars(s);
14	printChars(sb);
15	}
16	}
$ javac Interf1.java
$ java Interf1 CharSeq: Hello [0] H [1] e [2] l [3] l [4] o CharSeq: World [0] W [1] o [2] r [3] l [4] d
$ javap java.lang.CharSequence Compiled from "CharSequence.java" public interface java.lang.CharSequence{ public abstract int length(); public abstract char charAt(int); public abstract java.lang.CharSequence subSequence(int, int); public abstract java.lang.String toString(); }

CharSequence is something called an interface. An interface is like an abstract class, but it has no implemented methods. Both String and StringBuffer implement the CharSeqence interface.

While you can only extend a single class, you can implement many interfaces.

interf/A.java
1	package interf;
2
3	public interface A {
4	int getA();
5	}
$ javac interf/A.java

interf/B.java
1	package interf;
2
3	public interface B {
4	int getB();
5	}
$ javac interf/B.java

Interf2.java
1	*import interf.;**
2
3	public class Interf2 implements A, B {
4	int a, b;
5	public int getA() { return a; }
6	public int getB() { return b; }
7	public Interf2(int a,int b) {
8	this.a = a;
9	this.b = b;
10	}
11	public static void main(String[] args) {
12	Interf2 i = new Interf2(2,3);
13	System.out.println("a="+i.getA()+", b="+i.getB());
14	}
15	}
$ javac Interf2.java
$ java Interf2 a=2, b=3