System.out.println 流程详细解释

在java中，最经常使用的是System.out.println() 来进行打印输出

println方法会自动调用toString方法，如果不重写方法，直接对一个对象进行打印，一定会输出对象的类型+@+内存地址值

具体过程：

println首先调用的是对象的valueOf方法，而valueOf方法则是调用了对象的toString()方法，toString()方法原来是返回对象的格式化信息的，所以如果不重写toString()，那么就会打印这串字符的哈希值，重写的话就能把重写后的格式打印出来。

参考链接
println()源码：这个也解释了为什么println后面要换行，因为不只是print(x),还有一个newLine方法，用锁锁起来

/**
 * Prints an Object and then terminate the line.  This method calls
 * at first String.valueOf(x) to get the printed object's string value,
 * then behaves as
 * though it invokes {@link #print(String)} and then
 * {@link #println()}.
 *
 * @param x  The {@code Object} to be printed.
 */
public void println(Object x) {
    String s = String.valueOf(x);
    synchronized (this) {
        print(s);
        newLine();
    }
}

我们继续看print()的源代码

public void print(String s) {
    write(String.valueOf(s));
}

String.valueOf()源码：

/**
 * Returns the string representation of the {@code Object} argument.
 *
 * @param   obj   an {@code Object}.
 * @return  if the argument is {@code null}, then a string equal to
 *          {@code "null"}; otherwise, the value of
 *          {@code obj.toString()} is returned.
 * @see     java.lang.Object#toString()
 */
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

这里obj.toString会调用toString()的方法

注意：这里return的时候还运用到了三元运算符的知识

x?y:z如果x==true，则结果为y，否则结果为z

return (obj == null) ? "null" : obj.toString();可以被理解为如果obj为空，那么结果就输出null字符串，否则钓鱼obj的toString 方法来打印

package operator;

public class Demo08 {
    public static void main(String[] args) {
        //x?y:z
        //如果x==true，则结果为y，否则结果为z
        int score = 80;
        String type = score < 60 ? "不及格":"及格"; //必须掌握，让代码更加精简，且容易掌握
        System.out.println(type);
    }
}

toString（）源码：

官方文档建议每一个子类都重写这个toString 方法：It is recommended that all subclasses override this method.

如果直接打印一个对象，发现并不是对象的类型+@+内存地址值，则其内部方法一定已经重写过toString()方法过了

/**
 * Returns a string representation of the object. In general, the
 * {@code toString} method returns a string that
 * "textually represents" this object. The result should
 * be a concise but informative representation that is easy for a
 * person to read.
 * It is recommended that all subclasses override this method.
 * <p>
 * The {@code toString} method for class {@code Object}
 * returns a string consisting of the name of the class of which the
 * object is an instance, the at-sign character `{@code @}', and
 * the unsigned hexadecimal representation of the hash code of the
 * object. In other words, this method returns a string equal to the
 * value of:
 * <blockquote>
 * <pre>
 * getClass().getName() + '@' + Integer.toHexString(hashCode())
 * </pre></blockquote>
 *
 * @return  a string representation of the object.
 */
public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}