Java Buffer duplicate() 方法及实例

java.nio.Buffer 类的 duplicate() 方法是用来创建一个新的缓冲区，共享这个缓冲区的内容。新的缓冲区的内容将是这个缓冲区的内容。这个缓冲区的内容的变化将在新的缓冲区中可见，反之亦然；两个缓冲区的位置、极限和标记值将是独立的。新缓冲区的容量、极限、位置和标记值将与这个缓冲区的相同。当且仅当这个缓冲区是直接的，新的缓冲区将是直接的；当且仅当这个缓冲区是只读的，它将是只读的。

语法

public abstract Buffer duplicate()

返回值： 该方法返回承载先前缓冲区内容的新缓冲区。

下面是一些例子来说明 duplicate() 方法。

例子1： 直接使用ByteBuffer

// Java program to demonstrate
// duplicate() method
  
import java.nio.*;
import java.util.*;
  
public class GFG {
  
    public static void main(String[] args)
    {
  
        // Declaring the capacity of the ByteBuffer
        int capacity = 4;
  
        // Creating the ByteBuffer
        try {
  
            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb1
                = ByteBuffer.allocate(capacity);
  
            // putting the int to byte typecast
            // value in ByteBuffer
            bb1.put((byte)20);
            bb1.put((byte)30);
            bb1.put((byte)40);
            bb1.put((byte)50);
            bb1.rewind();
  
            // print the Original ByteBuffer
            System.out.println("Original ByteBuffer:  "
                               + Arrays.toString(bb1.array()));
  
            // Creating a duplicate copy of ByteBuffer
            // using duplicate() method
            ByteBuffer bb2 = bb1.duplicate();
  
            // print the duplicate copy of ByteBuffer
            System.out.print("\nDuplicate ByteBuffer: "
                             + Arrays.toString(bb2.array()));
        }
  
        catch (IllegalArgumentException e) {
  
            System.out.println("Exception thrown : " + e);
        }
  
        catch (ReadOnlyBufferException e) {
  
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出:

Original ByteBuffer:  [20, 30, 40, 50]

Duplicate ByteBuffer: [20, 30, 40, 50]

实例2： 使用只读的ByteBuffer

// Java program to demonstrate
// duplicate() method
  
import java.nio.*;
import java.util.*;
  
public class GFG {
  
    public static void main(String[] args)
    {
  
        // Declaring the capacity of the ByteBuffer
        int capacity = 4;
  
        // Creating the ByteBuffer
        try {
  
            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb1
                = ByteBuffer.allocate(capacity);
  
            // putting the int to byte typecast
            // value in ByteBuffer
            bb1.put((byte)20);
            bb1.put((byte)30);
            bb1.put((byte)40);
            bb1.put((byte)50);
            bb1.rewind();
  
            // print the Original ByteBuffer
            System.out.println("Original ByteBuffer:  "
                               + Arrays.toString(bb1.array()));
  
            // Creating a read-only copy of ByteBuffer
            // using asReadOnlyBuffer() method
            ByteBuffer readonly = bb1.asReadOnlyBuffer();
  
            // print the read-only copy of ByteBuffer
            System.out.print("\nRead-only ByteBuffer:  ");
            while (readonly.hasRemaining())
                System.out.print(readonly.get() + ", ");
            System.out.println("");
  
            // Rewinding the readonly ByteBuffer
            readonly.rewind();
  
            // Creating a duplicate copy of ByteBuffer
            // using duplicate() method
            ByteBuffer bb2 = readonly.duplicate();
  
            // print the duplicate copy of ByteBuffer
            System.out.print("\nDuplicate copy of "
                             + "read-only ByteBuffer:  ");
  
            while (bb2.hasRemaining())
                System.out.print(bb2.get() + ", ");
            System.out.println("");
        }
  
        catch (IllegalArgumentException e) {
  
            System.out.println("Exception thrown : " + e);
        }
  
        catch (ReadOnlyBufferException e) {
  
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出:

Original ByteBuffer:  [20, 30, 40, 50]

Read-only ByteBuffer:  20, 30, 40, 50, 

Duplicate copy of read-only ByteBuffer:  20, 30, 40, 50,

参考资料: https://docs.oracle.com/javase/9/docs/api/java/nio/Buffer.html#duplicate-