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import org.junit.Test;/** * @author CH * @create 2021 上午 11:23 */public class IDEADebug {    @Test    public void testStringBuffer(){        String str = null;        StringBuffer sb = new StringBuffer();        sb.append(str);//把null当4个字母添加进去        System.out.println(sb.length());//4        System.out.println(sb);//"null"        StringBuffer sb1 = new StringBuffer(str);//抛异常NullPointerException空指针异常        System.out.println(sb1);//    }}

 问题：

为什么 sb.append(str)后的str.length是4?

 进入StringBuffer()方法：

下一行：

 进入super.append(str);

进入return appendNull();

 进入StringBuffer（）

.length所以是空指针异常

附录：

/**  * Constructs a string buffer with no characters in it and  * the specified initial capacity.  *  * @param      capacity  the initial capacity.  * @exception  NegativeArraySizeException  if the {@code capacity}  *               argument is less than {@code 0}.  */ public StringBuffer(int capacity) {     super(capacity); } /**  * Constructs a string buffer initialized to the contents of the  * specified string. The initial capacity of the string buffer is  * {@code 16} plus the length of the string argument.  *  * @param   str   the initial contents of the buffer.  */ public StringBuffer(String str) {     super(str.length() + 16);     append(str); }

StringBuffer类 

StringBuffer 类的常用方法