字符所在的索引

可以使用charAt()方法从String对象中获取指定索引处的字符。索引值是从零开始的。以下代码在“zyiz.net”字符串中打印索引值和每个索引处对应的字符：

public class Main {
  public static void main(String[] args) {
    String str = "zyiz.net";

    // Get the length of string zyiz.net
    int len = str.length();

    // Loop through all characters and print their indexes
    for (int i = 0; i < len; i++) {
      System.out.println(str.charAt(i) + "  has  index   " + i);
    }

  }
}

上面的代码生成以下结果。

Y  has  index   0
I  has  index   1
I  has  index   2
B  has  index   3
A  has  index   4
I  has  index   5
.  has  index   6
C  has  index   7
O  has  index   8
M  has  index   9

测试字符串是否为空

测试String对象是否为空，空字符串的长度为零。有三种方法可以检查空字符串：

• isEmpty() 方法.
• equals() 方法.
• 获取字符串的长度，并检查它是否为零。

以下代码显示如何使用三种方法：

public class Main {
  public static void main(String[] args) {
    String str1 = "Hello";
    String str2 = "";
    // Using the isEmpty() method
    boolean empty1 = str1.isEmpty(); // Assigns false to empty1
    boolean empty2 = str2.isEmpty(); // Assigns true to empty1

    // Using the equals() method
    boolean empty3 = "".equals(str1); // Assigns false to empty3
    boolean empty4 = "".equals(str2); // Assigns true to empty4

    // Comparing length of the string with 0
    boolean empty5 = str1.length() == 0; // Assigns false to empty5
    boolean empty6 = str2.length() == 0; // Assigns true to empty6

  }
}

更改字符的大小写

要将字符串的内容转换为小写和大写，请分别使用toLowerCase()和toUpperCase()方法。

String str1 = new String("Hello"); // str1  contains "Hello" 
String str2 = str1.toUpperCase();   // str2 contains "HELLO" 
String str3 = str1.toLowerCase();   // str3 contains "hello"