Buat kelas baru pada package, dan pastekan kode ini

public PlainDocument getOnlyDigit(){
PlainDocument filterDigit = new PlainDocument(){
public void insertString(int offs, String str, AttributeSet a)throws BadLocationException{
StringBuffer buffer = new StringBuffer();
int s= 0;
char[]dataInput = str.toCharArray();
for (int i = 0; i < dataInput.length; i++){
boolean isOnllyDigit = Character.isDigit(dataInput[i]);
if(isOnllyDigit == true){
dataInput[s] = dataInput[i];
s++;
}
}
buffer.append(dataInput, 0, s);
super.insertString(offs, new String(buffer), a);
}
};
return filterDigit;
}

untuk importnya gunakan:

import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;