Android: ClassCastException for Button class

Question

I'm having problem using my custom Button class.

I've tried changing the xml layout from <Button/> to <gButton/>, but that hasn't worked.

I've also tried casting Button page1 = (gButton) findView...., in Line 15 but for some reason that doesn't work (you can cast a child class, right?)

The error I keep getting from LogCat is:

Caused by: java.lang.ClassCastException: android.widget.Button
     at flash.tut.ButtonPage.onCreate(ButtonPage.java:15)

Any help is much appreciated.

Here's my button view:

public class ButtonPage extends Activity{
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.button);

         gButton page1 = (gButton) findViewById(R.id.Button01); //<---LogCat: ClassCastException
         page1.setOnClickListener(new View.OnClickListener() {
              public void onClick(View view) {
                   Intent mI = new Intent(view.getContext(), Page1.class);
                   startActivityForResult(mI, 0);
              }
         });
     }
}

and my gButton which extends the button class:

public class gButton extends Button{
    private static Context con;

    public gButton(){this("Blank");}

    public gButton(String name){
        this(name, R.color.text_color);
    }

    public gButton(String name, int col) {
        this(name, col, con);
    }

    public gButton(String name, int col, Context context) {
        super(context);   //necessary context...dont know why.
        setBackgroundResource(R.drawable.icon);
        setTextColor(col);
        setText(name);
        setTextSize(14);
    }
}

And finally my xml layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

 <Button android:text="Page 1"
        android:id="@+id/Button01"
        android:layout_width="250px"
        android:textSize="18px"
        android:layout_height="55px">
    </Button>

 <Button android:text="Page 2"
        android:id="@+id/Button02"
        android:layout_width="250px"
      android:textSize="18px"
        android:layout_height="55px">
    </Button>

 <Button android:text="Page 3"
        android:id="@+id/Button03"
        android:layout_width="250px"
      android:textSize="18px"
        android:layout_height="55px">
    </Button>

<Button android:text="ListView Page"
        android:id="@+id/ButtonList"
        android:layout_width="250px"
      android:textSize="18px"
        android:layout_height="55px">
    </Button>

</LinearLayout>

nostra13 · Accepted Answer · 2011-11-27 19:08:46Z

2

To cast button to gButton you should declare buttons like this:

<com.example.gButton 
    android:text="Page 3"
    android:id="@+id/Button03"
    android:layout_width="250px"
    android:textSize="18px"
    android:layout_height="55px">
</com.example.gButton>

but you should put you package instead of "com.example".

And your gButton class has to have next constructor:

package com.example;

public class gButton extends Button{
    ...

    public gButton(Context context, AttributeSet attrs) { // this constructor will be called by inflater for creating object instance
        super(context, attrs);
    }

    ...

}

Then you can cast your button as you wrote in your question. But note that your constructors will be not called.

answered Nov 27, 2011 at 19:08

nostra13

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1 Comment

tetris11 Over a year ago

thank you, very useful and explicit answer. I'll try this tomorrow before confirming answer.

rfsk2010 · Accepted Answer · 2011-11-27 16:16:16Z

1

Button01 is a button while you are trying to cast a button to a gbutton.You can't cast an object into a derived object unless the object IS a derived object. For example, your have a base class called Product, and it is extended by CartItem.

Now, also imagine that the Product has other derived classes besides CartItem, such as PhoneItem, and MailOrderItem.

You can't do this: Product prod = new Product(); CartItem cart = (CartItem)prod.

Why?

Because "prod" isn't a CartItem any more than it is a PhoneItem and a MailOrderItem. You created it as a "Product" item, and it will ALWAYS be a Product item no matter how you try to cast it. You can force it to be a MailItem or a PhoneItem.

However, if you did the following, it work would: CartItem cart = new CartItem(); Product prod = (Product)cart; CartItem cart2 = (CartItem)prod;

Or

Product prod = new CartItem(); // (implicit cast to a Product) CartItem cart2 = (CartItem)prod;

Why does this work? Because a CartItem IS ALSO a Product (although not all products are cart items). Even though you casted it to a Product, it is still, and will ALWAYS be, a CartItem. The only difference is that the current handle to the CartItem happens to be a Product handle, giving you more limited access to all the features your CartItem can have. When you try to cast it back from a Product to a CartItem, the run-time engine says, "hey, can I cast this target object into the type they want?" At that point, it looks at the target object and finds out that the object, while currently being referenced as a Product, is REALLY a CartItem that has been upcast. Therefore, it is safe to cast it back to the CartItem it originally was. Once again, the point is that once you have created it as a CartItem, it is ALWAYS a CartItem, no matter how many times you cast it up or down. However, in the example you gave, your product item was never more than a simple Product. It was never a CartItem to begin with and couldn't be casted.

answered Nov 27, 2011 at 16:16

rfsk2010

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2 Comments

tetris11 Over a year ago

so you're saying there's no way I can possibly implement my custom button class, because casting just flat out wont work?

adamp Over a year ago

No, he's saying that there's no way that casting a Button to a gButton will work. ;) Try specifying gButtons instead of Buttons in your layout. There are two ways to do this: fully qualify the package name in the XML element e.g.: <com.your.package.gButton> or use <view class="com.your.package.gButton"> You're going to need the standard View constructor that takes the arguments (Context, AttributeSet) in order to use it in an XML layout.

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