首页 > 代码库 > 第六条---消除过期的对象的引用
第六条---消除过期的对象的引用
看下面的关于栈的程序：
package com.duo.month10day25;
import java.util.Arrays;
import java.util.EmptyStackException;
public class StackTest {
private Object[] elements;
private int size = 0;
private static final int DEFAULT_INITIAL_CAPACITY = 16;
public StackTest() {
elements = new Object[DEFAULT_INITIAL_CAPACITY];
}
public void push(Object e) {
ensureCapacity();
elements[size++] = e;
}
public Object pop() {
if (size == 0) {
throw new EmptyStackException();
}
return elements[--size];
}
private void ensureCapacity() {
if (elements.length == size) {
elements = Arrays.copyOf(elements, 2 * size + 1);
}
}
}
进入栈的源代码查看下Java是如何实现栈的：
/*
* Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.util;
/**
* The <code>Stack</code> class represents a last-in-first-out
* (LIFO) stack of objects. It extends class <tt>Vector</tt> with five
* operations that allow a vector to be treated as a stack. The usual
* <tt>push</tt> and <tt>pop</tt> operations are provided, as well as a
* method to <tt>peek</tt> at the top item on the stack, a method to test
* for whether the stack is <tt>empty</tt>, and a method to <tt>search</tt>
* the stack for an item and discover how far it is from the top.
* <p>
* When a stack is first created, it contains no items.
*
* <p>A more complete and consistent set of LIFO stack operations is
* provided by the {@link Deque} interface and its implementations, which
* should be used in preference to this class. For example:
* <pre> {@code
* Deque<Integer> stack = new ArrayDeque<Integer>();}</pre>
*
* @author Jonathan Payne
* @since JDK1.0
*/
public
class Stack<E> extends Vector<E> {
/**
* Creates an empty Stack.
*/
public Stack() {
}
/**
* Pushes an item onto the top of this stack. This has exactly
* the same effect as:
* <blockquote><pre>
* addElement(item)</pre></blockquote>
*
* @param item the item to be pushed onto this stack.
* @return the <code>item</code> argument.
* @see java.util.Vector#addElement
*/
public E push(E item) {
addElement(item);
return item;
}
/**
* Removes the object at the top of this stack and returns that
* object as the value of this function.
*
* @return The object at the top of this stack (the last item
* of the <tt>Vector</tt> object).
* @throws EmptyStackException if this stack is empty.
*/
public synchronized E pop() {
E obj;
int len = size();
obj = peek();
removeElementAt(len - 1);
return obj;
}
/**
* Looks at the object at the top of this stack without removing it
* from the stack.
*
* @return the object at the top of this stack (the last item
* of the <tt>Vector</tt> object).
* @throws EmptyStackException if this stack is empty.
*/
public synchronized E peek() {
int len = size();
if (len == 0)
throw new EmptyStackException();
return elementAt(len - 1);
}
/**
* Tests if this stack is empty.
*
* @return <code>true</code> if and only if this stack contains
* no items; <code>false</code> otherwise.
*/
public boolean empty() {
return size() == 0;
}
/**
* Returns the 1-based position where an object is on this stack.
* If the object <tt>o</tt> occurs as an item in this stack, this
* method returns the distance from the top of the stack of the
* occurrence nearest the top of the stack; the topmost item on the
* stack is considered to be at distance <tt>1</tt>. The <tt>equals</tt>
* method is used to compare <tt>o</tt> to the
* items in this stack.
*
* @param o the desired object.
* @return the 1-based position from the top of the stack where
* the object is located; the return value <code>-1</code>
* indicates that the object is not on the stack.
*/
public synchronized int search(Object o) {
int i = lastIndexOf(o);
if (i >= 0) {
return size() - i;
}
return -1;
}
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1224463164541339165L;
}
pop方法里面有个：removeElementAt这样的方法，再点击开来查看：
/**
* Deletes the component at the specified index. Each component in
* this vector with an index greater or equal to the specified
* {@code index} is shifted downward to have an index one
* smaller than the value it had previously. The size of this vector
* is decreased by {@code 1}.
*
* <p>The index must be a value greater than or equal to {@code 0}
* and less than the current size of the vector.
*
* <p>This method is identical in functionality to the {@link #remove(int)}
* method (which is part of the {@link List} interface). Note that the
* {@code remove} method returns the old value that was stored at the
* specified position.
*
* @param index the index of the object to remove
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index >= size()})
*/
public synchronized void removeElementAt(int index) {
modCount++;
if (index >= elementCount) {
throw new ArrayIndexOutOfBoundsException(index + " >= " +
elementCount);
}
else if (index < 0) {
throw new ArrayIndexOutOfBoundsException(index);
}
int j = elementCount - index - 1;
if (j > 0) {
System.arraycopy(elementData, index + 1, elementData, index, j);
}
elementCount--;
elementData[elementCount] = null; /* to let gc do its work */
}注意最后一行代码：
elementData[elementCount] = null; /* to let gc do its work */
看他的注释，而我们前面的代码有这样的代码吗？
1， 如果一个栈先是增长，然后再收缩，那么在栈中弹出来的对象将不会被当做垃圾回收，即使使用栈的程序不再引用这些对象，它们也不会被回收，因为，在栈的内部保存了对象的过期引用（obsolete reference），过期引用指的就是永远不会再被解除的引用。
2， 在stack中解决过期引用的方法就是在把对象推出栈的时候，设置栈为null。
3， 自行管理内存的类都有可能出现内存溢出问题。
4， 缓存也是具有内存溢出问题
5， 还有就是监听器和其他回调
6， 在缓存中，只要外部仍然有对象的引用，那么请使用WeakHashMap来代表缓存，这样就能管理内存。
7， 同理监听器和回调。保存成WeakHashMap中的键。
第六条---消除过期的对象的引用
声明：以上内容来自用户投稿及互联网公开渠道收集整理发布，本网站不拥有所有权，未作人工编辑处理，也不承担相关法律责任，若内容有误或涉及侵权可进行投诉： 投诉/举报 工作人员会在5个工作日内联系你，一经查实，本站将立刻删除涉嫌侵权内容。