java만으로는 어렵습니다. 운영체제가 윈도우라면 윈도우의 api 를 이용해야 합니다. 문제는 윈도우 api는 c/c++로 되어 있기 때문에 java(정확히는 jdk)만으로는 어렵고 jni를 이용해서 api를 호출해야 합니다.

바로 jni를 이용해도 되지만 편리하게 jna 라이브러리를 이용할 수 있습니다.

윈도우 핸들을 얻기 위해서는 FindWindow 함수를 사용하면 됩니다. https://msdn.microsoft.com/en-us/library/windows/desktop/ms633499(v=vs.85).aspx

윈도우의 크기및 위치를 얻을때 아래의 함수를 이용합니다.

GetWindowRect https://msdn.microsoft.com/ko-kr/library/windows/desktop/ms633519(v=vs.85).aspx

GetClientRect https://msdn.microsoft.com/ko-kr/library/windows/desktop/ms633503(v=vs.85).aspx

아래는 샘플입니다.

• build.gradle

plugins {
    id 'java'
    id 'application'
}

mainClassName = 'yhjung.Main'
group 'yhjung.com'
version '1.0-SNAPSHOT'

sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    testCompile group: 'junit', name: 'junit', version: '4.12'
    // https://mvnrepository.com/artifact/net.java.dev.jna/jna
    compile group: 'net.java.dev.jna', name: 'jna', version: '4.5.2'
    // https://mvnrepository.com/artifact/net.java.dev.jna/jna-platform
    compile group: 'net.java.dev.jna', name: 'jna-platform', version: '4.5.2'

}

• Main.java

package yhjung;

import java.util.Arrays;
import com.sun.jna.*;
import com.sun.jna.platform.win32.WinDef.HWND;
import com.sun.jna.win32.*;

public class Main {
    public interface User32 extends StdCallLibrary {
        User32 INSTANCE = (User32) Native.loadLibrary("user32", User32.class,
                W32APIOptions.DEFAULT_OPTIONS);

        HWND FindWindow(String lpClassName, String lpWindowName);

        int GetWindowRect(HWND handle, int[] rect);
    }

    public static int[] getRect(String windowName) throws WindowNotFoundException,
            GetWindowRectException {
        HWND hwnd = User32.INSTANCE.FindWindow(null, windowName);
        if (hwnd == null) {
            throw new WindowNotFoundException("", windowName);
        }

        int[] rect = {0, 0, 0, 0};
        int result = User32.INSTANCE.GetWindowRect(hwnd, rect);
        if (result == 0) {
            throw new GetWindowRectException(windowName);
        }
        return rect;
    }

    @SuppressWarnings("serial")
    public static class WindowNotFoundException extends Exception {
        public WindowNotFoundException(String className, String windowName) {
            super(String.format("Window null for className: %s; windowName: %s",
                    className, windowName));
        }
    }

    @SuppressWarnings("serial")
    public static class GetWindowRectException extends Exception {
        public GetWindowRectException(String windowName) {
            super("Window Rect not found for " + windowName);
        }
    }

    public static void main(String[] args) {
        String windowName = "Cmder";
        int[] rect;
        try {
            rect = Main.getRect(windowName);
            System.out.printf("The corner locations for the window \"%s\" are %s",
                    windowName, Arrays.toString(rect));
        } catch (Main.WindowNotFoundException e) {
            e.printStackTrace();
        } catch (Main.GetWindowRectException e) {
            e.printStackTrace();
        }
    }
}