Java ByteBuffer compact()方法及实例

java.nio.ByteBuffer 类的 compact() 方法用于压缩给定的缓冲区。

缓冲区的当前位置和其极限之间的字节（如果有的话）被复制到缓冲区的开头。也就是说，索引p=position()的字节被复制到索引0，索引p+1的字节被复制到索引1，以此类推，直到索引limit()-1的字节被复制到索引n=limit()-1-p，然后缓冲区的位置被设置为n+1，其极限被设置为其容量。如果定义了标记，则被丢弃。

缓冲区的位置被设置为被复制的字节数，而不是零，这样在调用这个方法后可以立即调用另一个相对的put方法。

在从缓冲区写入数据后调用这个方法，以防写入不完整。

语法：

public abstract ByteBuffer compact()

返回值： 该方法返回新的ByteBuffer，其内容与此缓冲区相同。

异常： 如果此缓冲区是只读的，该方法会抛出ReadOnlyBufferException。
以下程序说明了compact()方法：

例子1：

// Java program to demonstrate
// compact() method
 
import java.nio.*;
import java.util.*;
 
public class GFG {
    public static void main(String[] args)
    {
 
        // Declaring the capacity of the ByteBuffer
        int capacity = 7;
 
        // Creating the ByteBuffer
 
        // creating object of ByteBuffer
        // and allocating size capacity
        ByteBuffer bb = ByteBuffer.allocate(capacity);
 
        // putting the int to byte typecast value in ByteBuffer
        bb.put((byte)20);
        bb.put((byte)30);
        bb.put((byte)40);
 
        // print the ByteBuffer
        System.out.println("Original ByteBuffer: "
                           + Arrays.toString(bb.array()));
 
        System.out.println("Position: " + bb.position());
 
        System.out.println("limit: " + bb.limit());
 
        // Creating a compacted  ByteBuffer of same ByteBuffer
        // using compact() method
        ByteBuffer cbb = bb.compact();
 
        // print the ByteBuffer
        System.out.println("\nCompacted ByteBuffer: "
                           + Arrays.toString(cbb.array()));
 
        System.out.println("Position: " + cbb.position());
 
        System.out.println("limit: " + cbb.limit());
 
        // putting the int to byte typecast value in compacted ByteBuffer
        cbb.put((byte)50);
 
        // print the ByteBuffer
        System.out.println("\nUpdated Compacted ByteBuffer: "
                           + Arrays.toString(cbb.array()));
        System.out.println("Position: " + cbb.position());
        System.out.println("limit: " + cbb.limit());
    }
}

输出

Original ByteBuffer: [20, 30, 40, 0, 0, 0, 0]
Position: 3
limit: 7

Compacted ByteBuffer: [0, 0, 0, 0, 0, 0, 0]
Position: 4
limit: 7

Updated Compacted ByteBuffer: [0, 0, 0, 0, 50, 0, 0]
Position: 5
limit: 7

实例2：

// Java program to demonstrate
// compact() method
 
import java.nio.*;
import java.util.*;
 
public class GFG {
    public static void main(String[] args)
    {
 
        // Declaring the capacity of the ByteBuffer
        int capacity = 5;
 
        // Creating the ByteBuffer
        try {
 
            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb = ByteBuffer.allocate(capacity);
 
            // putting the int to byte typecast value in ByteBuffer
            bb.put((byte)20);
            bb.put((byte)30);
            bb.put((byte)40);
            bb.rewind();
 
            // Creating a read-only copy of ByteBuffer
            // using asReadOnlyBuffer() method
            ByteBuffer bb1 = bb.asReadOnlyBuffer();
 
            // print the ReadOnlyBuffer
            System.out.print("ReadOnlyBuffer ByteBuffer: ");
            while (bb1.hasRemaining())
                System.out.print(bb1.get() + ", ");
            System.out.println("");
 
            // print the Position of ByteBuffer bb
            System.out.println("\nPosition: " + bb.position());
 
            // print the Limit of ByteBuffer bb
            System.out.println("\nlimit: " + bb.limit());
 
            // Creating a compacted  ByteBuffer of same ReadOnlyBuffer
            // using compact() method
            System.out.println("\nTrying to compact the ReadOnlyBuffer bb1");
            ByteBuffer rbb = bb1.compact();
        }
 
        catch (IllegalArgumentException e) {
 
            System.out.println("Exception throws " + e);
        }
 
        catch (ReadOnlyBufferException e) {
 
            System.out.println("Exception throws " + e);
        }
    }
}

输出

ReadOnlyBuffer ByteBuffer: 20, 30, 40, 0, 0, 

Position: 0

limit: 5

Trying to compact the ReadOnlyBuffer bb1
Exception throws java.nio.ReadOnlyBufferException