Android: How can I validate EditText input?

Question

Why don't you use TextWatcher ?

Since you have a number of EditText boxes to be validated, I think the following shall suit you :

1. Your activity implements android.text.TextWatcher interface
2. You add TextChanged listeners to you EditText boxes

txt1.addTextChangedListener(this);
txt2.addTextChangedListener(this);
txt3.addTextChangedListener(this);

3. Of the overridden methods, you could use the afterTextChanged(Editable s) method as follows

@Override
public void afterTextChanged(Editable s) {
    // validation code goes here
}

The Editable s doesn't really help to find which EditText box's text is being changed. But you could directly check the contents of the EditText boxes like

String txt1String = txt1.getText().toString();
// Validate txt1String

in the same method. I hope I'm clear and if I am, it helps! :)

EDIT: For a cleaner approach refer to Christopher Perry's answer below.

TextWatcher is a bit verbose for my taste, so I made something a bit easier to swallow:

public abstract class TextValidator implements TextWatcher {
    private final TextView textView;

    public TextValidator(TextView textView) {
        this.textView = textView;
    }

    public abstract void validate(TextView textView, String text);

    @Override
    final public void afterTextChanged(Editable s) {
        String text = textView.getText().toString();
        validate(textView, text);
    }

    @Override
    final public void beforeTextChanged(CharSequence s, int start, int count, int after) { /* Don't care */ }

    @Override
    final public void onTextChanged(CharSequence s, int start, int before, int count) { /* Don't care */ }
}

Just use it like this:

editText.addTextChangedListener(new TextValidator(editText) {
    @Override public void validate(TextView textView, String text) {
       /* Validation code here */
    }
});

If you want nice validation popups and images when an error occurs you can use the setError method of the EditText class as I describe here

In order to reduce the verbosity of the validation logic I have authored a library for Android. It takes care of most of the day to day validations using Annotations and built-in rules. There are constraints such as @TextRule, @NumberRule, @Required, @Regex, @Email, @IpAddress, @Password, etc.,

You can add these annotations to your UI widget references and perform validations. It also allows you to perform validations asynchronously which is ideal for situations such as checking for unique username from a remote server.

There is a example on the project home page on how to use annotations. You can also read the associated blog post where I have written sample codes on how to write custom rules for validations.

Here is a simple example that depicts the usage of the library.

@Required(order = 1)
@Email(order = 2)
private EditText emailEditText;

@Password(order = 3)
@TextRule(order = 4, minLength = 6, message = "Enter at least 6 characters.")
private EditText passwordEditText;

@ConfirmPassword(order = 5)
private EditText confirmPasswordEditText;

@Checked(order = 6, message = "You must agree to the terms.")
private CheckBox iAgreeCheckBox;

The library is extendable, you can write your own rules by extending the Rule class.

This was nice solution from here

InputFilter filter= new InputFilter() { 
    public CharSequence filter(CharSequence source, int start, int end, Spanned dest, int dstart, int dend) { 
        for (int i = start; i < end; i++) { 
            String checkMe = String.valueOf(source.charAt(i));

            Pattern pattern = Pattern.compile("[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789_]*");
            Matcher matcher = pattern.matcher(checkMe);
            boolean valid = matcher.matches();
            if(!valid){
                Log.d("", "invalid");
                return "";
            }
        } 
        return null; 
    } 
};

edit.setFilters(new InputFilter[]{filter});