How to call methods of a Service from activity?

Here is an example that might help
Server.java:

package com.example.bindservice.binder;

import java.text.SimpleDateFormat;
import java.util.Date;

import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.IBinder;

public class Server extends Service {

    IBinder mBinder = new LocalBinder();

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    public class LocalBinder extends Binder {
        public Server getServerInstance() {
            return Server.this;
        }
    }

    public String getTime() {
        SimpleDateFormat mDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return mDateFormat.format(new Date());
    }
}

Client.java

package com.example.bindservice.binder;

import android.app.Activity;
import android.content.ComponentName;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import com.example.bindservice.binder.Server.LocalBinder;

public class Client extends Activity {

    boolean mBounded;
    Server mServer;
    TextView text;
    Button button;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        text = (TextView)findViewById(R.id.text);
        button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                text.setText(mServer.getTime());
            }
        });
    }

    @Override
    protected void onStart() {
        super.onStart();

        Intent mIntent = new Intent(this, Server.class);
        bindService(mIntent, mConnection, BIND_AUTO_CREATE);
    };

    ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceDisconnected(ComponentName name) {
            Toast.makeText(Client.this, "Service is disconnected", 1000).show();
            mBounded = false;
            mServer = null;
        }

        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            Toast.makeText(Client.this, "Service is connected", 1000).show();
            mBounded = true;
            LocalBinder mLocalBinder = (LocalBinder)service;
            mServer = mLocalBinder.getServerInstance();
        }
    };

    @Override
    protected void onStop() {
        super.onStop();
        if(mBounded) {
            unbindService(mConnection);
            mBounded = false;
        }
    };
}

There is sample code for this right in the Service documentation, under "Local Service Sample":

http://developer.android.com/reference/android/app/Service.html#LocalServiceSample

Also for those people suggesting aidl -- if your service and client are all part of your own .apk and running in the same process (the default behavior), there is no need for aidl; it is just additional complexity that doesn't give you anything.

How do you call a normal Java method?

A obj = new A();
obj.method();

Service is a Java class. So how would you call a service method?

serviceObj.method();

Now the real question is how do you create a Service object?

Service serviceObj = new Service();

Definitely not.

In Android, Service is a System component that is created, destroyed and managed by Android OS.
For creating a service object you need IBinder.

This is how you can get a Service object from IBinder.

Once you have a hold of serviceObject. It will work like any normal Java object.
The above thing explained in the figure is called Binding a Service.

Binding makes possible to observe a background service from an Activity. With binding, we can communicate in both ways ie Activity<--->Service.
Prateek Yadav has already provided an excellent code snippet. You can use that.

Few things to keep in mind

• Never forget to unbind the service else you will lead to ResourceLeak.
• You can call startService(intent) and bindService(mIntent, mConnection, BIND_AUTO_CREATE) in any order. Binding and Starting a service are two independent things.

One way to do this is by defining an interface with Android's AIDL and making use of the Binder subsystem to perform IPC. There is a great set of instructions at the link I posted. I'd start there and then post here if you have questions. Despite being a pretty complex topic (IPC) Android and the Binder do a really good job of making it pretty dead simple (at least to get started, I'm sure you could make it complicated if you wanted to ;-) )

Edit As pointed out in the comments, this is unnecessary if the Service and the client are running in the same process. Unless you specify otherwise, this is the default. However, it still works regardless, it just adds a bit more complexity.