java中==比较的是两个字符串的是否同一个对象
*****
```
// "static void main" must be defined in a public class.
public class Main {
public static void main(String[] args) {
// initialize
String s1 = "Hello World";
System.out.println("s1 is \"" + s1 + "\"");
String s2 = s1;
System.out.println("s2 is another reference to s1.");
String s3 = new String(s1);
System.out.println("s3 is a copy of s1.");
// compare using '=='
System.out.println("Compared by '==':");
// true since string is immutable and s1 is binded to "Hello World"
System.out.println("s1 and \"Hello World\": " + (s1 == "Hello World"));
// true since s1 and s2 is the reference of the same object
System.out.println("s1 and s2: " + (s1 == s2));
// false since s3 is refered to another new object
System.out.println("s1 and s3: " + (s1 == s3));
// compare using 'equals'
System.out.println("Compared by 'equals':");
System.out.println("s1 and \"Hello World\": " + s1.equals("Hello World"));
System.out.println("s1 and s2: " + s1.equals(s2));
System.out.println("s1 and s3: " + s1.equals(s3));
// compare using 'compareTo'
System.out.println("Compared by 'compareTo':");
System.out.println("s1 and \"Hello World\": " + (s1.compareTo("Hello World") == 0));
System.out.println("s1 and s2: " + (s1.compareTo(s2) == 0));
System.out.println("s1 and s3: " + (s1.compareTo(s3) == 0));
}
}
```
*****
```
// "static void main" must be defined in a public class.
public class Main {
public static void main(String[] args) {
String s1 = "Hello World";
// 1. concatenate
s1 += "!";
System.out.println(s1);
// 2. find
System.out.println("The position of first 'o' is: " + s1.indexOf('o'));
System.out.println("The position of last 'o' is: " + s1.lastIndexOf('o'));
// 3. get substring
System.out.println(s1.substring(6, 11));
}
}
```
