解读StringBuffer,StringBuilder,String区别

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源码级StringBuffer和StringBuilder以及String区别

作为一个面试常问的问题：
首先看一下源码：

public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

这是源码从这里可以看出两者都可序列化，CharSequence是一个字符序列，从这里我们
看不出来区别

看下面的代码可以看出：
1、StringBuffe方法前都加了synchronized所以第一个区别就是StringBuffer是线程安全的，适合多线程环境。

StringBuffer源码

@Override
    public synchronized int length() {
        return count;
    }

    @Override
    public synchronized int capacity() {
        return value.length;
    }


    @Override
    public synchronized void ensureCapacity(int minimumCapacity) {
        super.ensureCapacity(minimumCapacity);
    }

    /**
     * @since      1.5
     */
    @Override
    public synchronized void trimToSize() {
        super.trimToSize();
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized void setLength(int newLength) {
        toStringCache = null;
        super.setLength(newLength);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }

StringBuilder的源码

 @Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

    /**
     * Appends the specified {@code StringBuffer} to this sequence.
     * 

     * The characters of the {@code StringBuffer} argument are appended,
     * in order, to this sequence, increasing the
     * length of this sequence by the length of the argument.
     * If {@code sb} is {@code null}, then the four characters
     * {@code "null"} are appended to this sequence.
     * 

     * Let n be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at index
     * k in the new character sequence is equal to the character at
     * index k in the old character sequence, if k is less than
     * n; otherwise, it is equal to the character at index k-n
     * in the argument {@code sb}.
     *
     * @param   sb   the {@code StringBuffer} to append.
     * @return  a reference to this object.
     */
    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }

    @Override
    public StringBuilder append(CharSequence s) {
        super.append(s);
        return this;
    }

    /**
     * @throws     IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(CharSequence s, int start, int end) {
        super.append(s, start, end);
        return this;
    }

2、
StringBuilder的包：

package java.lang;

StringBuffer的包：

package java.lang;
import java.util.Arrays;

从这句话可以看出：两个类都是属于lang下的，但是StringBuffer中导入了Arrays的

StringBuffer中多出有一个这样的属性

/**
     * A cache of the last value returned by toString. Cleared
     * whenever the StringBuffer is modified.
     */
    private transient char[] toStringCache;

解释一下这个代码的意思：
注释的意思是：toString返回的最后一个值的缓冲，当StringBuffer被修改时清除

transient修饰的变量值不参与序列化。

这个不重要，既然看到这儿了，我们在看一下String的特殊之处：

来，上源码：

public final class String
    implements java.io.Serializable, Comparable, CharSequence {
    /** The value is used for character storage. */
    private final char value[];

精华全在上边了：
1、首先多实现了Comparable接口，没有实现AbstractStringBuilder
2、低层value数组不可变，那我们看一下StringBuffer和StringBuilder

abstract class AbstractStringBuilder implements Appendable, CharSequence {
    /**
     * The value is used for character storage.
     */
    char[] value;

这是另一个区别是String是不可变的。