String和StringBuffer和StringBuilder

public class StringTest {
    public static void main(String[] args) {
        //string不可变
        String str = "aaabbb";
        String str2 = str.substring(2,5);
        System.out.println(str);
        System.out.println(str2);
        //做string比较的时候用equal不用==
        String str11 = "hello java";
        String str22 = "hello";
        String str33 = " java";
        String str44 = str22+str33;
        System.out.println(str11.equals(str44));
        System.out.println(str11== str44);

        //StringBuffer和StringBuilder都是可变的，StringBuilder线程不安全，效率高（一般使用）
        StringBuffer sb1 = new StringBuffer("abcd");
        StringBuilder sb2 =new StringBuilder("abcd");
        System.out.println(sb1.hashCode());
        sb1.setCharAt(0,'A');
        System.out.println(sb1);
        System.out.println(sb1.hashCode());
    }
}