java mocking paths

CommandLineInterface.java

package com.mastermind;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.List;

public class CommandLineInterface {
PrintStream output = System.out;

    public void setOutput(PrintStream output) {
        this.output = output;
    }
    ...
}

CommandLineInterfaceTest.java

public class CommandLineInterfaceTest {
  private CommandLineInterface cli = new CommandLineInterface();
  ..
  
  @Test
  public void itDisplaysAWelcomeMessage() {
      MockOutputStream out = new MockOutputStream();
      PrintStream printStream = new PrintStream(out);
      cli.setOutput(printStream);
      cli.displayWelcomeMessage();
      assertEquals("", out.getStringHistory());
  }
}

gistfile1.txt

The MockOutputStream does work, it can mocks the output, when I remove line 8 from the CommandLineInterface test, the output is printed to the console. However, the assertion is failing with

Expected: 
Actual : []

So, the StringHistory array isn't getting populated.

MockOutputStream.java

package com.mastermind;

import java.io.IOException;
import java.io.OutputStream;
import java.util.ArrayList;

public class MockOutputStream extends OutputStream {
    ArrayList<String> stringHistory = new ArrayList<String>();

    @Override
    public void write(int b) throws IOException {
    }

    public void write(String message) throws IOException {
        stringHistory.add(message);
    }

    public ArrayList<String> getStringHistory() {
        System.out.println("hi");
        return stringHistory;
    }
}