See Also:

java.io.PrintStream.println()

java.io.PrintStream.println(boolean)

java.io.PrintStream.println(char)

java.io.PrintStream.println(char [])

java.io.PrintStream.println(double)

java.io.PrintStream.println(float)

java.io.PrintStream.println(int)

java.io.PrintStream.println(long)

java.io.PrintStream.println(java.lang.Object)

java.io.PrintStream.println(java.lang.String)

上面是println的所有方法,對于你傳遞的Course對象,println只能認知為Object,然后我們來看

void java.io.PrintStream.println(Object x) 方法:

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 print(String) and then ?println().

Parameters:

x ? The Object to be printed.

從上面可知,方法會首先調用一個String.valueOf(x)的方法講其變成一個字符串然后再輸出,x是你傳遞進去的值.

所以你再自己去看String.valueOf(x)方法的內容吧