Java 中的字符串理解

1public class StringCreation {
2    public static void main(String[] args) {
3        // Method 1: String Literal (Recommended)
4        // Stored in the String Pool
5        String s1 = "Java"; 
6        String s2 = "Java"; // Reuses the same object as s1
7
8        // Method 2: Using 'new' (Avoid unless necessary)
9        // Forces a new object on the Heap, outside the Pool
10        String s3 = new String("Java"); 
11
12        // Comparing references (addresses), NOT content
13        System.out.println("s1 == s2: " + (s1 == s2)); // true (Same pool object)
14        System.out.println("s1 == s3: " + (s1 == s3)); // false (Different objects)
15    }
16}

1public class StringImmutability {
2    public static void main(String[] args) {
3        String greeting = "Hello";
4        
5        // We attempt to modify the string
6        greeting.concat(", World"); 
7        System.out.println("After concat (ignored): " + greeting);
8
9        // We must reassign the variable to store the change
10        greeting = greeting.concat(", World"); 
11        System.out.println("After reassignment: " + greeting);
12    }
13}

1public class StringConcat {
2    public static void main(String[] args) {
3        String firstName = "Ada";
4        String lastName = "Lovelace";
5        int year = 1815;
6
7        // Concatenating strings and numbers
8        String profile = firstName + " " + lastName + " (Born: " + year + ")";
9        
10        System.out.println(profile);
11    }
12}

1public class StringFormatting {
2    public static void main(String[] args) {
3        String product = "Laptop";
4        double price = 999.9985;
5        int stock = 5;
6
7        // Using String.format (Static method)
8        String label = String.format("Item: %s | Price: $%.2f | Stock: %d", product, price, stock);
9        
10        // Using .formatted (Instance method - Java 15+)
11        String modernLabel = "Item: %s | Price: $%.2f | Stock: %d".formatted(product, price, stock);
12
13        System.out.println(label);
14        System.out.println(modernLabel);
15    }
16}

1public class StringComparison {
2    public static void main(String[] args) {
3        String literal = "password";
4        String input = new String("password"); // Simulates user input
5
6        // WRONG: Comparing memory addresses
7        if (literal == input) {
8            System.out.println("Access Granted (==)");
9        } else {
10            System.out.println("Access Denied (==)"); // This executes
11        }
12
13        // CORRECT: Comparing text content
14        if (literal.equals(input)) {
15            System.out.println("Access Granted (.equals)"); // This executes
16        }
17    }
18}