How to prevent the object to exceed the limits of the screen?

Question

I started programming a little game where an object moves across the screen via the arrow keys on the keyboard. The difficulty I found is the fact that the object surpasses the JFrame, that is, there is nothing to block the passage at the border of JFrame.

I would like to know how I can solve this problem, or at least an indication of a path I should take to achieve success in this goal.

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Jogo1 extends JFrame{ //Herda para usar JFrame

String n = "0";
JLabel labelFlappy = new JLabel(n);

int posPrincX = 300;
int posPrincY = 300;

public Jogo1(){
    editarJanela();
    editarComponentes();
    addMovimento();
}

public void addMovimento(){
    addKeyListener(new KeyListener(){ //Teclado
        public void keyPressed(KeyEvent e) {
            System.out.println(e.getKeyCode()); //Exibe código da tecla pressionada
            if(e.getKeyCode() == 38){
                posPrincY -= 20;
            }
            if(e.getKeyCode() == 40){
                posPrincY += 20;
            }
            if(e.getKeyCode() == 37){
                posPrincX -= 20;
            }
            if(e.getKeyCode() == 39){
                posPrincX += 20;
            }
            labelFlappy.setBounds(posPrincX, posPrincY, 180, 90); //Atualiza posição
        }
        public void keyReleased(KeyEvent e) {   

        }           
        public void keyTyped(KeyEvent e) {

        }
    });
    addMouseListener(new MouseListener() {

        public void mouseClicked(MouseEvent e) { //Ao clicar no objeto em questão
            posPrincX = (int)(Math.random() * 650);
            posPrincY = (int)(Math.random() * 700);
            labelFlappy.setBounds(posPrincX, posPrincY, 180, 90);
        }
        public void mouseEntered(MouseEvent e) {

        }
        public void mouseExited(MouseEvent e) {

        }
        public void mousePressed(MouseEvent e) {

        }
        public void mouseReleased(MouseEvent e) {

        }   
    });
}

public void editarComponentes(){
    labelFlappy.setBounds(posPrincX, posPrincY, 180, 90);
}

public void editarJanela(){
    setDefaultCloseOperation(EXIT_ON_CLOSE); //Encerra ao fechar 
    setSize(750, 700); //Tamanho 
    setLocationRelativeTo(null); //Centraliza
    setVisible(true); //Torna visível
    setLayout(null); //Permite redimensionamento de cada componente
    setResizable(false); //Impossibilita o redimensionamento pelo usuário
    setTitle("Magisterix"); //Título

    add(labelFlappy);
}

public static void main(String[] args){
    new Jogo1();
}}

Answer 1

To prevent moving components from exceeding the screen area, you can use a method that checks whether new X and Y positions, summed with the width and overall height of the component, exceed the screen size in both directions, and when they exceed, no longer move the component or apply some negative value so it does not get stuck in the corner. The method to verify this, based on your code, can be more or less like this:

private boolean excedeuAreaDaTela(int posX, int posY) {
    return posX < 0 || posX + 180 > 750 || posY < 0 || posY + 90 > 700;
}

To use, just involve with a conditional the line you change the position of the component:

if (!excedeuAreaDaTela(posPrincX, posPrincY)) {
    labelFlappy.setBounds(posPrincX, posPrincY, 180, 90); // Atualiza posição
}

When the method returns true, means that if the component moves to the given coordinates, it will exceed the size you set for the screen, the conditional will not allow movement in this case.

Answer 2

I managed to do so:

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Jogo1 {

    private static final int KEY_CODE_DOWN = 38;
    private static final int KEY_CODE_UP = 40;
    private static final int KEY_CODE_LEFT = 37;
    private static final int KEY_CODE_RIGHT = 39;

    private static final int DISTANCIA_PASSO = 20;
    private static final int LARGURA_INICIAL_TELA = 750;
    private static final int ALTURA_INICIAL_TELA = 700;
    private static final int POSICAO_X_INICIAL = 300;
    private static final int POSICAO_Y_INICIAL = 300;
    private static final int LARGURA_FLAPPY_INICIAL = 180;
    private static final int ALTURA_FLAPPY_INICIAL = 90;

    private int posicaoX;
    private int posicaoY;

    public Jogo1() {
        JFrame tela = new JFrame("Magisterix");
        tela.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // Encerra ao fechar.
        tela.setSize(LARGURA_INICIAL_TELA, ALTURA_INICIAL_TELA); // Ajusta o tamanho.
        tela.setLocationRelativeTo(null); // Centraliza.
        tela.setVisible(true); // Torna visível.
        tela.setResizable(false); // Impossibilita o redimensionamento pelo usuário.
        tela.setLayout(null); // Permite redimensionamento de cada componente.

        JLabel labelFlappy = new JLabel("0");
        labelFlappy.setOpaque(true);
        labelFlappy.setBackground(Color.YELLOW);
        tela.add(labelFlappy);
        posicaoX = POSICAO_X_INICIAL;
        posicaoY = POSICAO_Y_INICIAL;
        labelFlappy.setBounds(posicaoX, posicaoY, LARGURA_FLAPPY_INICIAL, ALTURA_FLAPPY_INICIAL);

        tela.addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                System.out.println(e.getKeyCode());
                if (e.getKeyCode() == KEY_CODE_DOWN) {
                    posicaoY -= DISTANCIA_PASSO;
                } else if (e.getKeyCode() == KEY_CODE_UP) {
                    posicaoY += DISTANCIA_PASSO;
                } else if (e.getKeyCode() == KEY_CODE_LEFT) {
                    posicaoX -= DISTANCIA_PASSO;
                } else if (e.getKeyCode() == KEY_CODE_RIGHT) {
                    posicaoX += DISTANCIA_PASSO;
                }
                posicaoX = limitar(0, posicaoX, tela.getContentPane().getWidth() - labelFlappy.getWidth());
                posicaoY = limitar(0, posicaoY, tela.getContentPane().getHeight() - labelFlappy.getHeight());
                labelFlappy.setBounds(posicaoX, posicaoY, labelFlappy.getWidth(), labelFlappy.getHeight());
            }
        });

        tela.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                posicaoX = (int) (Math.random() * (tela.getContentPane().getWidth() - labelFlappy.getWidth()));
                posicaoY = (int) (Math.random() * (tela.getContentPane().getHeight() - labelFlappy.getHeight()));
                labelFlappy.setBounds(posicaoX, posicaoY, labelFlappy.getWidth(), labelFlappy.getHeight());
            }
        });
    }

    private static int limitar(int min, int meio, int max) {
        return meio < min ? min : meio > max ? max : meio;
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(Jogo1::new);
    }
}

The trick is to limit posPrincX and posPrincY the size of the window (or posicaoX and posicaoY, since I renamed the variables). For this, I use the method limitar(int, int, int).

I made some other changes to your code:

• Only create, interact, use and access Swing components within the EDT. For this, I use the EventQueue.invokeLater(Runnable). See more about this in that question and in that reply.

• Prefer to use composition rather than inheritance, and therefore not inherit from JFrame. I talk about it in that reply. I also address this issue (and many other things) in that other answer.

• Use magic numbers is not good programming practice. I made your code without using magic numbers and also in a way that if the screen size or label size change, it will not get messy and will adjust to the change alone.

• I painted the JLabel yellow so you can see exactly where it is and what its size.

• I changed the name of some variables to be more representative.

• Use KeyAdapter and MouseAdapter instead of implementing directly KeyListener and MouseListener not to need to have that lot of empty methods.