如何判断按钮视图是否存在于java/android中

Question

I find this hard to believe that i can not find this answer anywhere. It does not seem like the code that i have works correctly for checking the existence of a button view. Here is what i have, any suggestions are greatly appreciated. Thanks!

Java file:

public class LayoutsActivity extends Activity {

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.main);
    ActivateButtons();
}

void ActivateButtons()
{
    Button mainLayoutButton = (Button) this.findViewById(R.id.MainLayout);
    if(mainLayoutButton != null)
    {
        mainLayoutButton.setOnClickListener(new View.OnClickListener() {    
            public void onClick(View v) {
                setContentView(R.layout.main);
                ActivateButtons();
            }
        });
    }

    Button linearLayoutButton = (Button) this.findViewById(R.id.LinearLayout);
    if(linearLayoutButton != null)
    {
        linearLayoutButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                setContentView(R.layout.mylinearlayout);
                ActivateButtons();
            }
        });
    }

    Button tableLayoutButton = (Button) this.findViewById(R.id.TableLayout);
        tableLayoutButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                setContentView(R.layout.mytablelayout);
                ActivateButtons();
            }
        });


    Button frameLayoutButton = (Button) this.findViewById(R.id.FrameLayout);
    if(frameLayoutButton != null)
    {
        frameLayoutButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                setContentView(R.layout.myframelayout);
                ActivateButtons();
            }
        });
    }
    Button relativeLayoutButton = (Button) this.findViewById(R.id.RelativeLayout);
    if(relativeLayoutButton != null)
    {
        relativeLayoutButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                setContentView(R.layout.myrelativelayout);
                ActivateButtons();
            }
        });
    }

    Button absoluteLayoutButton = (Button) this.findViewById(R.id.AbsoluteLayout);
    if(absoluteLayoutButton != null)
    {
         absoluteLayoutButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                setContentView(R.layout.myabsolutelayout);
                ActivateButtons();
            }
        }); 
    }

    Button DynamicLayoutButton = (Button) this.findViewById(R.id.DynamicLayout);
    if(DynamicLayoutButton != null)
    {
        DynamicLayoutButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                LinearLayout dynamicLayout = new LinearLayout(LayoutsActivity.this);
                dynamicLayout.setOrientation(LinearLayout.VERTICAL);
                LinearLayout.LayoutParams size=new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);

                final TextView mytext= new TextView(LayoutsActivity.this);
                mytext.setText("This is the dynamic layout");
                mytext.setTextSize(20);
                mytext.setGravity(Gravity.CENTER);

                dynamicLayout.addView(mytext, size);
                /* Button b= new Button(LayoutsActivity.this);
                b.setText("Click Here to go to Main Layout");
                dynamicLayout.addView(b, size); */

                Button b= new Button(LayoutsActivity.this);
                b.setText("Main Layout");
                b.setId(R.id.DynamicLayout);
                dynamicLayout.addView(b, size);

                //Step L
                Button linear = new Button(LayoutsActivity.this);
                b.setText("Linear Layout");
                b.setId(R.id.LinearLayout);
                dynamicLayout.addView(linear, size);

                Button table = new Button(LayoutsActivity.this);
                table.setText("Table Layout");
                table.setId(R.id.LinearLayout);
                dynamicLayout.addView(table, size);

                ActivateButtons();

                setContentView(dynamicLayout);
            }
        });
    }
}

}

XML file: I have one for each layout but all are similar

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

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello" />

    <Button
        android:id="@+id/LinearLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Linear layout" />
        <!-- The layout lays out its children next to each other (horizontally or vertically)-->
    <Button
        android:id="@+id/TableLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Table Layout" />
        <!-- The layout represents a table like structure where there are columns and rows for the children-->
    <Button
        android:id="@+id/FrameLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Frame Layout" />
        <!-- The layout places the children on top of one another-->
    <Button
        android:id="@+id/RelativeLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Relative Layout" />
        <!-- The layout positions its children relative to each other-->
    <Button
        android:id="@+id/AbsoluteLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Absolute Layout" />
        <!-- The layout has children with absolute coordinates-->
    <Button
        android:id="@+id/DynamicLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Dynamic Layout" />
        <!-- The layout creates a second, new page -->
    <Button
        android:id="@+id/MainLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Main Layout" />
        <!-- The layout returns to the original layout which seems to be similar to the linear layout-->
</LinearLayout>

Answer 1

I'm a little confused. If your button is in the XML layout, how could it never not exist? And if it wasn't in the XML, your R.id.MainLayout would not compile. Unless you're saying MainLayout is some layout and not a button. Which is also gonna cause a problem because you're casting it to a button.. Does this code compile and run?

Answer 2

I need to update a dynamically created button in an array without the app crashing if it doesn't exist. I have implemented it like this. This works for me.

    for(int x=0; x<numberOfRecords; x++) {

        try {
            buttonsArray[x].setEnabled(false);
        } catch (Exception e) {
            // code to deal with error here
        }
    }