System.out输入参数

Question

I have a test file and according to it I need to build my program the test file is below. However, I confused by s1.showDetails(System.out); I have never met System.out in parameter can anyone help. What to do with it??? when I am trying to write showDetails() the compiler writes mistake. my student code is beneath this Thank you in advance!

import java.util.*;
public class Q2 {
    public static void main(String [] args)
    {
        // Start on section A
        System.out.println("Question 2");
        System.out.println("Start on part A");
        Student s1 = new Student("John", "Smith", 42);
        s1.showDetails(System.out);
        Course cs = new Course("Computer science");
    }
}

public class Student {
    private String name;
    private String familyName;
    private int moduleMark;
    private int total;
    protected Student(String name, String familyName, int moduleMark)
    {
        this.name = name;
        this.familyName = familyName;
        this.moduleMark = moduleMark;
    }

    public String getName()
    {
        return name;
    }

    public String getFamilyName()
    {
        return familyName;
    }

    public int getModuleMark()
    {
        return moduleMark;
    }

    public String showDetails()
    {
        return (this.name + " " + this.familyName + " " + moduleMark + total);
        //print(name);
    }
}

Answer 1

System.out is a variable like every other variable.

System is a class

out is a public static variable inside System of type PrintStream . So you can access it with System.out

So System.out.println(..) is just a call to the println(..) function of a PrintStream

So your function should look like:

public String showDetails(PrintStream stream) {
 ...
}

Answer 2

The error essentially means that the compiler didn't find the method with the name showDetails that takes an argument of type PrintStream . You have no need to pass the System.out to the showDetails() method. The correct way writing the showDetails() is below. Read about System.out .

public void showDetails()
{
   System.out.println(this.name + " " + this.familyName + " " + moduleMark + total);
}

Answer 3

The error pretty much describes what the problem is.

The method showDetails() in the type Student is not applicable for the arguments (PrintStream) at Q2.main(Q2.java:9)

The program tries to call your method with System.out which happens to be a PrintStream. So, change your showDetails to

public String showDetails(PrintStream out) {
    out.println(this.name + " " + this.familyName + " " + moduleMark + total);
}

This allows the tester (I assume there is a program which tests your assignment for correctness) to give you any PrintStream, either System.out or any other PrintStream.

Answer 4

Your program can't compile.

Your main is using a method named showDetails that takes a PrintStream as a parameter (System.out). And you are defining only a method named showDetails without parameter.

Look at the doc of System.out . It sa field like any other of the class System. Indeed it's somewhat special as it is static but that doesn't change the game that much...

So write a method with the correct parameter list and you will get closer.

public String showDetails(PrintStream stream) {
 //your code here
}

showDetails should write to the stream passed in parameter.

While you are at learning programming. Try to separate query from command . It's a good principle : a method should do one thing : either do something to your current object using a parameter or answer a query about the state of your current object. Not both. Here you return a string and require a stream... You should better split that into 2 methods : one to get the string and then as second one to call stream.write on that string.

public String toString() {
 //your code here
}

public void showDetails(PrintStream stream) {
 //your code here
}

Answer 5

Since the print is commented out from your showDetails message there are 2 obvious solutions

1. The showDetails message is not meant to print out anything (despite the name) and you should just print it out in the main method

    System.out.println( s1.showDetails() ); 

2. The showDetails message should print the String to any PrintStream , and then you have to adjust both the signature of that method as well as the implementation

    public String showDetails(PrintStream stream) { stream.println( ... ); }