共同点

他们都是可变的，在每次进行修改操作时，都不会产生新的对象，所以在进行修改的时候，尽量使用这两种类型的字符串

不同点

StringBuffer在单线程中效率高
StringBuilder用于多线程确保安全性

测试代码

public class test {
    public static void main(String[] args) {
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append("a");
        System.out.println(stringBuilder.hashCode());

        stringBuilder.append("b");
        System.out.println(stringBuilder.hashCode());
        stringBuilder.append("c");
        System.out.println(stringBuilder.hashCode());
        System.out.println(stringBuilder);
        String string = new String();
        string += "1";
        System.out.println(string.hashCode());
        string += "2";
        System.out.println(string.hashCode());
        string += "3";
        System.out.println(string.hashCode());
        System.out.println(string);

        StringBuffer stringBuffer = new StringBuffer();
        stringBuffer.append("a");
        System.out.println(stringBuffer.hashCode());

        stringBuffer.append("b");
        System.out.println(stringBuffer.hashCode());
        stringBuffer.append("c");
        System.out.println(stringBuffer.hashCode());
    }
}