1. String s1 = "Hello";
2. String s2 = "Hello";
3. String s3 = new String("Hello");
4. String s4 = new String("Hello");

 public class String50 {
     public static void main(String[] args) {
         String s1 = "Hello";
         String s2 = "Hello";
         String s3 = new String("Hello");
         String s4 = new String("Hello");
 
         System.out.println("s1 == s2: " + (s1 == s2));
         System.out.println("s1 == s3: " + (s1 == s3));
         System.out.println("s1 == s4: " + (s1 == s4));
         System.out.println("s3 == s4: " + (s3 == s4));
 
         System.out.println("s1.equals(s2): " + (s1.equals(s2)));
         System.out.println("s1.equals(s3): " + (s1.equals(s3)));
         System.out.println("s1.equals(s4): " + (s1.equals(s4)));
         System.out.println("s3.equals(s4): " + (s3.equals(s4)));
 
     }
 }

□ 実行結果

$ java String50
s1 == s2: true
s1 == s3: false
s1 == s4: false
s3 == s4: false
s1.equals(s2): true
s1.equals(s3): true
s1.equals(s4): true
s3.equals(s4): true

 public class String52 {
     public static void main(String[] args) {
         String s = "Hello Java World";
 
         System.out.println("                              0123456789012345");
         System.out.println("s                           = " + s                 );
         System.out.println("s.length()                  = " + s.length()        );
         System.out.println("s.charAt(6)                 = " + s.charAt(6)       );
         System.out.println("s.indexOf(\"x\")              = " + s.indexOf("x")    );
         System.out.println("s.indexOf(\"l\")              = " + s.indexOf("l")    );
         System.out.println("s.indexOf(\"l\", 6)           = " + s.indexOf("l", 6) );
         System.out.println("s.matches(\".*Java.*\")       = " + s.matches(".*Java.*") );
         System.out.println("s.substring(2, 5)           = " + s.substring(2, 5) );
         System.out.println("s.substring(11, 15)         = " + s.substring(11, 15));
         System.out.println("s.substring(11, 16)         = " + s.substring(11, 16));
         System.out.println("s.substring(11)             = " + s.substring(11)    );
         System.out.println("s.toLowerCase()             = " + s.toLowerCase()   );
         System.out.println("s.toUpperCase()             = " + s.toUpperCase()   );
         System.out.println("s.replaceAll(\"Hello\", \"Hi\") = " + s.replaceAll("Hello", "Hi"));
         String[] sa = s.split(" ");
         for (int i = 0; i < sa.length; i++) {
             System.out.println("s.split(\" \")[" + i + "]             = " + sa[i]);
         }
     }
 }

□ 実行結果

$ java String52
                              0123456789012345
s                           = Hello Java World
s.length()                  = 16
s.charAt(6)                 = J
s.indexOf("x")              = -1
s.indexOf("l")              = 2
s.indexOf("l", 6)           = 14
s.matches(".*Java.*")       = true
s.substring(2, 5)           = llo
s.substring(11, 15)         = Worl
s.substring(11, 16)         = World
s.substring(11)             = World
s.toLowerCase()             = hello java world
s.toUpperCase()             = HELLO JAVA WORLD
s.replaceAll("Hello", "Hi") = Hi Java World
s.split(" ")[0]             = Hello
s.split(" ")[1]             = Java
s.split(" ")[2]             = World

 public class String60 {
     public static void main(String[] args) {
         int i = -1;
         while (/* ここにコードを書いてください。*/) {
             System.out.println(i);
         } 
     }
 }

□ 実行結果

$ java String60 "hello world" l
2
3
9

□ 実行結果

$ java String601 "HELLO world" l
2
3
9

 public class String61 {
     public static void main(String[] args) {
         int i = -1;
         while ( /* ここにコードを書いてください。*/ ) {
 
             // ここにコードを書いてください。
 
         } 
     }
 }

□ 実行結果

$ java String61 "Hello World" l
He l lo World
Hel l o World
Hello Wor l d