Introduction to Java Strings: Creation and Common Methods

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質問：

Introduce Java Strings and give examples

回答：

Java Strings are a sequence of characters used to represent text in Java programming. They are objects of the String class, which is part of the java.lang package. Strings in Java are immutable, meaning that once a String object is created, its value cannot be changed. However, you can create new String objects based on existing ones.

Creating Strings

You can create a String in Java in two main ways:

  1. Using String Literals:

    String str1 = "Hello, World!";

  2. Using the new Keyword:

    String str2 = new String("Hello, World!");

Common String Methods

Java provides a variety of methods to manipulate and interact with strings. Here are some commonly used methods:

  1. Length:

    String str = "Hello";
int length = str.length(); // Returns 5

  2. Substring:

    String str = "Hello, World!";
String sub = str.substring(7, 12); // Returns "World"

  3. Concatenation:

    String str1 = "Hello";
String str2 = "World";
String result = str1 + " " + str2; // Returns "Hello World"

  4. Character at a Specific Index:

    String str = "Hello";
char ch = str.charAt(1); // Returns 'e'

  5. String Comparison:

    String str1 = "Hello";
String str2 = "hello";
boolean isEqual = str1.equals(str2); // Returns false
boolean isEqualIgnoreCase = str1.equalsIgnoreCase(str2); // Returns true

  6. Finding a Character or Substring:

    String str = "Hello, World!";
int index = str.indexOf("World"); // Returns 7

  7. Replacing Characters:

    String str = "Hello, World!";
String newStr = str.replace("World", "Java"); // Returns "Hello, Java!"

  8. Converting to Uppercase/Lowercase:

    String str = "Hello, World!";
String upper = str.toUpperCase(); // Returns "HELLO, WORLD!"
String lower = str.toLowerCase(); // Returns "hello, world!"

  9. Trimming Whitespace:

    String str = "   Hello, World!   ";
String trimmed = str.trim(); // Returns "Hello, World!"

Example Program

Here’s a simple Java program that demonstrates some of the string operations:

public class StringExample {
    public static void main(String[] args) {
        String greeting = "Hello, World!";
        
        // Length of the string
        System.out.println("Length: " + greeting.length());
        
        // Substring
        System.out.println("Substring: " + greeting.substring(7, 12));
        
        // Concatenation
        String name = "Java";
        String message = greeting + " Welcome to " + name + "!";
        System.out.println("Message: " + message);
        
        // Character at index
        System.out.println("Character at index 1: " + greeting.charAt(1));
        
        // String comparison
        System.out.println("Equals 'Hello, World!': " + greeting.equals("Hello, World!"));
        
        // Replace
        System.out.println("Replaced: " + greeting.replace("World", "Java"));
        
        // Uppercase
        System.out.println("Uppercase: " + greeting.toUpperCase());
        
        // Trim
        String spacedString = "   Hello, World!   ";
        System.out.println("Trimmed: '" + spacedString.trim() + "'");
    }
}

Conclusion

Java Strings are a powerful and flexible way to handle text in your applications. Understanding how to create and manipulate strings is fundamental for any Java programmer.

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