截取字符串：substring(args1,args2);
substring(args1,args2);
如：截取“HelloWorld”字符串的Hello；
String string = "HelloWorld";
System.out.print(string.substring(0,5));
比较两个字符串是否相等：equals();
equals比较的是值，==比较的是内存
sting1.equals(string2);
如：比较string1和string2
String string1 = "Hello";
String string2 = "Hello";
//new 的变量都储存在堆内存里
String string3 = new String("Hello");
System.out.println(string1 == string2);
System.out.print(string1.equals(string2));
System.out.print(string1.equals(string2));
System.out.println(string1 == string3);
