Intercept system.out in java

I have a method that has system.out and it's used in main. How can I intercept that print and assign it to a String variable? I also do not want it to be printed.

public void print()
System.out.println("hello");

---main---
print(); // need to intercept this 
String str = print(); // need assign the contents of the print() to str and not show the contents of print in the console

Edit: Due to some restrictions, I cannot create .txt and I cannot change the code of the method. I need to make all the changes in main

Solution

You can call System.setOut() to change the PrintStream used by System.out. You probably also want to call setErr() to the same PrintStream.

To illustrate, let us use a standard "Hello World" program.

public static void main(String[] args) {
    System.out.println("Hello World");
}

Output

Hello World

We now replace the output print stream with a print stream that captures all the output in a buffer. We do keep a copy of the original print stream so we can print something for real at the end.

public static void main(String[] args) {
    PrintStream oldSysOut = System.out;
    ByteArrayOutputStream outBuf = new ByteArrayOutputStream();
    try (PrintStream sysOut = new PrintStream(outBuf, false, StandardCharsets.UTF_8)) {
        System.setOut(sysOut);
        System.setErr(sysOut);
        
        // Normal main logic goes here
        System.out.println("Hello World");
    }
    String output = new String(outBuf.toByteArray(), StandardCharsets.UTF_8);
    oldSysOut.print("Captured output: \"" + output + "\"");
}

Output

Captured output: "Hello World
"

As can be seen here, all the output was captured, including the linebreak from the println() call.