Java Buffer clear()方法及示例

java.nio.ByteBuffer类的 clear() 方法被用来清除这个缓冲区。位置被设置为零，极限被设置为容量，并且标记被丢弃。在使用一连串的通道读或放操作来填充这个缓冲区之前调用这个方法。

例如

buf.clear();     // Prepare buffer for reading
in.read(buf);    // Read data

这个方法实际上并不清除缓冲区中的数据，但它被命名为仿佛清除了数据，因为它最常被用于可能是这种情况的场合。

语法

public Buffer clear()

返回值： 该方法返回这个缓冲区。

下面是说明clear()方法的例子。

例子 1 :

// Java program to demonstrate
// clear() method
  
import java.nio.*;
import java.util.*;
  
public class GFG {
  
    public static void main(String[] args)
    {
  
        try {
  
            byte[] barr = { 10, 20, 30, 40 };
  
            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb = ByteBuffer.wrap(barr);
  
            // Typecasting ByteBuffer into Buffer
            Buffer bb1 = (Buffer)bb;
  
            // try to set the position at index 2
            bb1.position(2);
  
            // Set this buffer mark position
            // using mark() method
            bb1.mark();
  
            // try to set the position at index 4
            bb1.position(4);
  
            // display position
            System.out.println("position before reset: "
                               + bb1.position());
  
            // try to call clear() to restore
            // to the position at index 0
            // by discarding the mark
            bb1.clear();
  
            // display position
            System.out.println("position after reset: "
                               + bb1.position());
        }
  
        catch (InvalidMarkException e) {
            System.out.println("new position is less than "
                               + "the position we marked before ");
            System.out.println("Exception throws: " + e);
        }
    }
}

输出:

position before reset: 4
position after reset: 0

例子 2 :

// Java program to demonstrate
// clear() method
  
import java.nio.*;
import java.util.*;
  
public class GFG {
  
    public static void main(String[] args)
    {
  
        try {
  
            byte[] barr = { 10, 20, 30, 40 };
  
            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb = ByteBuffer.wrap(barr);
  
            // Typecasting ByteBuffer into Buffer
            Buffer bb1 = (Buffer)bb;
  
            // try to set the position at index 2
            bb1.position(3);
  
            // display position
            System.out.println("position before clear: "
                               + bb1.position());
  
            // try to call clear() to restore
            // to the position at index 0
            // by discarding the mark
            bb1.clear();
  
            // display position
            System.out.println("position after clear: "
                               + bb1.position());
        }
  
        catch (InvalidMarkException e) {
            System.out.println("new position is less than "
                               + "the position we marked before ");
            System.out.println("Exception throws: " + e);
        }
    }
}

输出:

position before clear: 3
position after clear: 0

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