Modern Android App Development with Java/

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Data Validation and Error Handling

Data Validation and Error Handling

This lesson will cover how to validate user input text and show error messages.

We'll cover the following...

View binding #

To interact with views we need to bind the view from XML to Java objects via the findViewById method.

Note: You can only start binding views after the activity has been created and the content view has been set.

Java
public class LoginActivity extends AppCompatActivity {
    private TextInputLayout textUsernameLayout;
    private TextInputLayout textPasswordInput;
    private Button loginButton;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        textUsernameLayout = findViewById(R.id.textUsernameLayout);
        textPasswordInput = findViewById(R.id.textPasswordInput);
        loginButton = findViewById(R.id.loginButton);
    }
}

Validation & error message #

To perform validation, we need to set a click listener on the login button via the setOnClickListener method, which accepts the OnClickListener interface as a parameter. When the login button is clicked, we execute our onLoginClicked method.

Java
public class LoginActivity extends AppCompatActivity {
    ...
    private Button loginButton;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        ...
        loginButton = findViewById(R.id.loginButton);
        loginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                LoginActivity.this.onLoginClicked();
            }
        });
    }
    
    private void onLoginClicked() {
        // not implemented yet
    }
}

Click listener can be slightly simplified to lambda because we use Java 8.

Java
loginButton = findViewById(R.id.loginButton);
loginButton.setOnClickListener(v -> onLoginClicked());

Now, we can implement ...