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String、StringBuffer和StringBuilder的区别

可变性

String是不可变的，它使用final关键词修饰--

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

StringBuffer和StringBuilder两者都是继承自AbstractStringBuilder,AbstractStringBuilder也是用字符数组保存char value[]当时没有用final修饰，所以这两种对象是可变的

public final class StringBuffer
    extends AbstractStringBuilder

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

线程安全性

String的对象不可变，也就是常量，所以线程安全
StringBuffer重写了AbstractStringBuilder的方法，比如append、insert等并且加了同步锁，所以是线程安全的

@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);
    }