javax.swing.UIManager.getIcon(Object key) returns null when String-keys are streamed from an ArrayList

What I want to achieve: I want to visualize some javax.swing.Icons from the javax.swing.UIManager. On the internet I've found a list of UIManager-keys, which will not only return Icons but also Strings, Colors and so on. So in this step I want to filter the list of keys so only the Icon-keys remain.

My Approach: I copied a list of UIManager keys into a textfile and included it as recource in my Java-Project. I successfully read the file so I split the file-content by lines and added them to an ArrayList of Strings. Now i wanted to stream the content of this ArrayList and filter the keys by wether the UIManager.getIcon(Object key)-Method returns null or not...

My Problem so far: the UIManager always returns null. I printed all the keys and the UIManager result to the console (see "Output / Test - stream keys" in my code). If i manually copy a key from the console (one that I know should work) and paste it into the exact same piece of code, it actually works (see "Output / Test - single Key" in my code).

Interesting Behavior shows when I append a String to the key that I want to print to the console (See the variable "suffix" under "Output / Test - stream Keys" in my code). If the variable suffix does not start with "\n", the following print-Method in the stream will only print the suffix and no longer show the other content. For example if I type String suffix = "test"; only "test" will be printed from the .forEach(key->System.out.println(... + key + suffix); However, this behavior does not show up in the "Output / Test - single Key"-Example.

I have no idea, what is going on or if the (in my opinion) strange behavior as anything to do with the problem. I appreciate any kind of help!

Piece from "UIManagerKeys.txt": Here are some keys for testing and reproducibility purposes...

FileView.computerIcon
FileView.directoryIcon
FileView.fileIcon
FileView.floppyDriveIcon
FileView.hardDriveIcon
FormattedTextField.background
FormattedTextField.border
windowText

My Code:

package main;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.util.ArrayList;
import java.util.Arrays;
import javax.swing.UIManager;

public class Main {

    public Main() {
    }

    public static void main(String args[]) {
        ArrayList<String> uiKeys = new ArrayList<>();
        String fileName = "recources/UIManagerKeys.txt";
        ClassLoader classLoader = new Main().getClass().getClassLoader();

        File file = new File(classLoader.getResource(fileName).getFile());

        // Check: is File found?
        System.out.println("File Found : " + file.exists());

        try {
            // Read File Content
            String content = new String(Files.readAllBytes(file.toPath()));

            // Split by line and collect
            String[] keys = content.split("\n");
            uiKeys.addAll(Arrays.asList(keys));
        } catch (IOException e) {
            System.out.println("Error: " + e);
        }

        // Output / Test - stream Keys
        System.out.println("Total Number of Keys: " + uiKeys.size());
        String suffix = ""; // Interesting behavior when String is not empty
        uiKeys.stream()
                .map(key -> key.replaceAll(" ", "").replaceAll("\n", "")) // Just to be sure
                .forEach(key -> System.out.println("IconFound: " + (UIManager.getIcon(key) != null) + "\tKey: " + key + suffix));

        // Output / Test - single Key
        System.out.println("\n");
        String key = "FileView.directoryIcon"; // Copied from console
        System.out.println("IconFound: " + (UIManager.getIcon(key) != null) + "\tKey: " + key + suffix);
    }
}

I want to visualize some javax.swing.Icons from the javax.swing.UIManager.

I copied a list of UIManager keys into a textfile

There is no need to create the text file. You just get all the properties from the UIManager and check if the Object is an Icon:

public static void main(String[] args) throws Exception
{
    UIDefaults defaults = UIManager.getLookAndFeelDefaults();

    for ( Enumeration enumm = defaults.keys(); enumm.hasMoreElements(); )
    {
        Object key = enumm.nextElement();
        Object value = defaults.get( key );

        if (value instanceof Icon)
            System.out.println( key );
    }
}

On the internet I've found a list of UIManager-keys,

Check out: UIManager Defaults for a little graphical tool that displays all the properties.

Your approach of reading the files seems a little complicated. Here is a simple approach to reading the lines of a file into a ListArray:

import java.io.*;
import java.util.*;

public class ReadFile
{
    public static void main(String [] args) throws Exception
    {
        ArrayList<String> lines = new ArrayList<String>();

        BufferedReader in = new BufferedReader( new FileReader( "ReadFile.java" ) );
        String line;

        while((line = in.readLine()) != null)
        {
            lines.add(line);
        }

        in.close();

        System.out.println(lines.size() + " lines read");
    }
}