Java String之迷惑

package com.bobo;

public class Test {

/**
* @param args
*/
public final String a;

public final String a1 = "a";

public Test() {
a = "a";
}

public static void main(String[] args) {
new Test().testA();

}

public void testA() {
String b = a + "b";
String b1 = a1 + "b";
String c = "ab";
System.out.println(c == b);//false
System.out.println(c == b1);//true
}
}