Java字符串拼接问题

public class StringTest {
    public static void main(String[] args) {
        String str1 = "abc";
        String str2 = new String("abc");
        String str3 = "ab";
        String str4 = "c";
        //String——>StringBuilder,调用append()
        //StringBuilder——>String:toString()
        //toString中有new String()
        String str5 = str3+str4;
        String str6 = str3+str4;
        String str7 = str3+"c";
        String str8 = "ab"+"c";
        /*
        * String类中只要有变量的加法运算，编译器都会将其优化为StringBuilder
        * 而后调用Aappend方法进行字符串拼接，而后调用toString()转为String（内部使用new String）
        *  public String toString() {
             return new String(value, 0, count);
           }
        * */
        System.out.println(str1==str2);//F
        System.out.println(str1==str5);//F
        System.out.println(str1==str6);//F
        System.out.println(str1==str7);//F
        System.out.println(str1==str8);//T
    }
}