看代码学Android ：如何判断SD卡是否可用?

package com.test;

//import java.net.URISyntaxException;

import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
//import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class TestActivity1 extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        Button btn = (Button) this.findViewById(R.id.button1);
        btn.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {

                // SD Card Ready
                if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {

                    Toast.makeText(getApplicationContext(), "SD Card Ready! Enjoy it!",
                             Toast.LENGTH_LONG).show();
                }
                else{
                    Toast.makeText(getApplicationContext(), "SD Card NOT Ready! Please insert it, try it agian!",
                             Toast.LENGTH_LONG).show();
                }

            }
        });
    }
}