String类型内存转换，你明白为什么吗？

package learnIng;public class StringTest {    public static void main(String[] args) {        String a = "a";        String b = "a";        String c = new String("a");        String d=c.intern();        System.out.println(a.equals(b));        System.out.println(a == b);        System.out.println("------------");        System.out.println(a.equals(c));        System.out.println(a == c);        System.out.println("------------");        System.out.println(a.equals(d));        System.out.println(a == d);    }}

实验结果：
true
true

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true
false

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true
true
能否知道为什么？