基本类型和引用类型

Table

1public class PrimitivesExample {
2    public static void main(String[] args) {
3        int population = 8000;
4        double price = 19.99;
5        boolean isActive = true;
6        char grade = 'A';
7
8        System.out.println("Population: " + population);
9        System.out.println("Price: " + price);
10    }
11}

1public class ReferenceExample {
2    public static void main(String[] args) {
3        // 'greeting' is the reference (holds the address)
4        // "Hello Java" is the object (stored separately in memory)
5        String greeting = "Hello Java";
6        
7        System.out.println(greeting);
8    }
9}

1public class PrimitiveCopy {
2    public static void main(String[] args) {
3        int a = 10;
4        int b = a;  // Copies the value 10 into b
5        
6        b = 55;     // Changes b only
7        
8        System.out.println("a: " + a); // a remains 10
9        System.out.println("b: " + b); // b is 55
10    }
11}

1public class ReferenceAlias {
2    public static void main(String[] args) {
3        // Create a modifiable string containing "Start"
4        StringBuilder ref1 = new StringBuilder("Start");
5        
6        // Copy the address from ref1 to ref2
7        StringBuilder ref2 = ref1;
8        
9        // Change the text using ref2
10        ref2.append("+End");
11        
12        // ref1 sees the change because it points to the same object
13        System.out.println("ref1: " + ref1); 
14    }
15}

1public class NullExample {
2    public static void main(String[] args) {
3        String message = null; // message holds no address
4        
5        // System.out.println(message.length()); // RUNTIME ERROR
6        
7        System.out.println("Message is: " + message);
8    }
9}