java中对象为null时的打印输出方式

 public static void main(String[] args) {
        String a = null;
        String b = "null";
        Object c = null;
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
    }

   public void println(String x) {
        synchronized (this) {
            print(x);
            newLine();
        }
    }
    public void print(String s) {
        write(String.valueOf(s));
    }
    public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

byte -> 0
short -> 0
int -> 0
long -> 0
char -> " "(看起来像是一个空格)
float -> 0.0
double -> 0.0
boolean -> false

String s = null;
try{
    s.toString();
} catch(NullPointerException e){
    e.printStackTrace();
}
System.out.println(s);

public void println(String x) {
    synchronized (this) {
        print(x);
        newLine();
    }
}

public void print(String s) {
    if (s == null) {
        s = "null";
    }
    write(s);
}

Integer i = null;
try{
    i.toString();
} catch(NullPointerException e){
    e.printStackTrace();
}
System.out.println(i);

public void println(Object x) {
    String s = String.valueOf(x);
    synchronized (this) {
        print(s);
        newLine();
    }
}

public void print(String s) {
    if (s == null) {
        s = "null";
    }
    write(s);
}

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

public void println(Object x) {
    synchronized (this) {
        print(x);
        newLine();
    }
}