为什么我们需要 System class 来调用 out.println() 方法？

Question

When we want to print line in console we write System.out.println(data), what will be difference if we use PrintStream object out and use out.println(data).

Thanks.

Answer 1

System.out is a static reference to the PrintStream "out". So there is no difference.

PrintStream out = System.out;
out.println("Badgers");

Answer 2

Why do we need System class to call out.println() method?

We don't. It is just a convenient way to get hold of the PrintStream object for writing to standard output.

... what will be difference if we use PrintStream object out and use out.println(data).

It depends on how you obtained the PrintStream object.

• If you originally got the PrintStream object from System.out or System.err , then there is no difference; eg the following have the same effect:

   public void sayHello(PrintStream ps) { ps.println("hello"); } sayHello(System.out); System.out.println("hello");

• If you created the PrintStream on a stream that opened yourself, the behaviour will be analogous, but the output will (most likely) go to a different place; eg

  sayHello(new PrintStream(new FileOutputStream("foo.txt"));

  will write "hello" to the named file.

There is nothing special about System.in/out/err . They are just public static final variables that are initialized to specific stream objects when the JVM starts. (OK, there is something slightly unusual about them... but it doesn't manifest here.)

Answer 3

There's absolutely no difference. From ProgrammerInterview.com :

Inside the System class is the declaration of out that looks like: public static final PrintStream out , and inside the Printstream class is a declaration of println() that has a method signature that looks like: public void println() .

PrintStream is a class which streams the data to the standard output. An association was made by adding the object which is static and final out in the System class. So we as programmers can use it as System the class name (System.out) out is static object of PrintStream class and println( ) is a method in PrintStream class which are overloaded methods. So we directly write System.out.println(data) .

Answer 4

System.out is a static PrintStream object which is bound to a special file of the System: the standard output. You can assign the reference of it to any other PrintStream variable.

Answer 5

None, but out is commonly used as a variable in a program so it might be ambiguous.

Answer 6

println() is a method in PrintStream class. out is the object of PrintStream class which is defined as static constant in System class with the address of console.

So, in whatever way we invoke that method it will be same.

But if you dont want to use System class, you need to create the PrintStream object by giving the standard output address.

Answer 7

The words System.out.println have been used so much in the past that every casual reader of Java code knows what they mean. Everyone writes it this way, and people got accustomed to it.

The code would probably be harder to read if you first assigned System.out to a local variable. This is because in every line you would have to ask yourself: “Well, something gets printed, but where ?”. The idiom System.out.println on the other hand answers that question. It clearly sticks out from the code as something being output (even if it is just for debugging purposes).

So yes, you can use a shortcut for System.out.println , but in most cases you shouldn't.