java - ByteBuffer类中的get方法为抽象的，为什么下面的程序可以直接使用？

Question

import java.nio.*;
import java.nio.channels.*;
import java.io.*;

public class GetChannel {
  private static final int BSIZE = 1024;
  public static void main(String[] args) throws Exception {
    // Write a file:
    FileChannel fc =
      new FileOutputStream("data.txt").getChannel();
    fc.write(ByteBuffer.wrap("Some text ".getBytes()));
    fc.close();
    // Add to the end of the file:
    fc =
      new RandomAccessFile("data.txt", "rw").getChannel();
    fc.position(fc.size()); // Move to the end
    fc.write(ByteBuffer.wrap("Some more".getBytes()));
    fc.close();
    // Read the file:
    fc = new FileInputStream("data.txt").getChannel();
    ByteBuffer buff = ByteBuffer.allocate(BSIZE);
    fc.read(buff);
    buff.flip();
    while(buff.hasRemaining())
      System.out.print((char)buff.get());         // 抽象方法为什么可以直接使用？
  }
}

Answer 1

public static ByteBuffer allocate(int capacity) {

    if (capacity < 0)
        throw new IllegalArgumentException();
    return new HeapByteBuffer(capacity, capacity);

}

allocate实例化返回的是个HeapByteBuffer。

class HeapByteBuffer extends ByteBuffer

所以此处是父类的抽象方法调用具体的子类实例化的方法

Answer 2

查看源码可以知道

ByteBuffer buff = ByteBuffer.allocate(BSIZE);
这一步实例化了ByteBuffer的子类HeapByteBuffer

 public static ByteBuffer allocate(int capacity) {
        if (capacity < 0)
            throw new IllegalArgumentException();
        return new HeapByteBuffer(capacity, capacity);
    }

而在HeapByteBuffer里面get（int i）方法被重写了

public byte get(int i) {
        return hb[ix(checkIndex(i))];
    }