总是有人说,打印一个对象的时候,会自动调用一个对象的toString方法,但是。。。
- 一个null,在被打印时,根本不会抛出NullPointerException,而是奇怪的打印出“null”。
@org.junit.Test
public void test4(){
Object string = null;
System.out.println(string);
}
运行结果:
- 这是因为System.out.println()方法,会进行判空操作:
/**
* Prints a string. If the argument is <code>null</code> then the string
* <code>"null"</code> is printed. Otherwise, the string's characters are
* converted into bytes according to the platform's default character
* encoding, and these bytes are written in exactly the manner of the
* <code>{@link #write(int)}</code> method.
*
* @param s The <code>String</code> to be printed
*/
public void print(String s) {
if (s == null) {
s = "null";
}
write(s);
}
/**
* Prints an object. The string produced by the <code>{@link
* java.lang.String#valueOf(Object)}</code> method is translated into bytes
* according to the platform's default character encoding, and these bytes
* are written in exactly the manner of the
* <code>{@link #write(int)}</code> method.
*
* @param obj The <code>Object</code> to be printed
* @see java.lang.Object#toString()
*/
public void print(Object obj) {
write(String.valueOf(obj));
}
跟进去valueOf吧:
/**
* Returns the string representation of the <code>Object</code> argument.
*
* @param obj an <code>Object</code>.
* @return if the argument is <code>null</code>, then a string equal to
* <code>"null"</code>; otherwise, the value of
* <code>obj.toString()</code> is returned.
* @see java.lang.Object#toString()
*/
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}