1.字符串的相加

String str1 = "abc";//长度不可改变
String str2 = "def";
String str3 = str1 + str2;//字符串变量相加浪费内存（中间的对象）
System.out.println(str3);//输出结果abcdef

2.StringBuilder的加字符

StringBuilder builder = new StringBuilder("java");
//把StringBuilder当做一个类，需要去新建一个类的实例也就是对象，而不是声明类型+直接赋值
builder.append("python");//builder后面要加上python
builder.append("C++");
builder.append("UI");
builder.append(3123);
System.out.println("builder="+builder);

//输出结果：builder=javapythonC++UI3123

3.StringBuffer的加字符(append

StringBuffer buffer = new StringBuffer("");//字符串为空
buffer.append("asd");
buffer.append(123456);
System.out.println("buffer="+buffer);

//输出结果：

buffer=asd123456

4.连续加字符

StringBuffer buffer2 = new StringBuffer("");
buffer2.append("asd").append("fdsf");
System.out.println("buffer2="+buffer2);

//输出结果：

buffer2=asdfdsf