Java NullPointerException – الكشف عن الخطأ وإصلاحه وأفضل الممارسات

public class Temp {

	public static void main(String[] args) {

		Temp t = initT();
		
		t.foo("Hi");
		
	}

	private static Temp initT() {
		return null;
	}

	public void foo(String s) {
		System.out.println(s.toLowerCase());
	}
}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:7)

public class Temp {

	public int x = 10;
	
	public static void main(String[] args) {

		Temp t = initT();
		
		int i = t.x;
		
	}

	private static Temp initT() {
		return null;
	}

}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:9)

public class Temp {

	public static void main(String[] args) {

		foo(null);

	}

	public static void foo(String s) {
		System.out.println(s.toLowerCase());
	}
}

public class Temp {

	public static void main(String[] args) {

		throw null;
	}

}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:5)

public class Temp {

	public static void main(String[] args) {

		int[] data = null;
		
		int len = data.length;
	}

}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:7)

public class Temp {

	public static void main(String[] args) {

		int[] data = null;
		
		int len = data[2];
	}

}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:7)

public class Temp {

	public static String mutex = null;
	
	public static void main(String[] args) {

		synchronized(mutex) {
			System.out.println("synchronized block");
		}		
	}
}

@RequestMapping(value = "/user", method = RequestMethod.POST)
	public String user(@Validated User user, Model model) {
		System.out.println("User Page Requested");
		System.out.println("User Name: "+user.getUserName().toLowerCase());
		System.out.println("User ID: "+user.getUserId().toLowerCase());
		model.addAttribute("userName", user.getUserName());
		return "user";
	}

HTTP Status 500 – Internal Server Error

Type Exception Report

Message Request processing failed; nested exception is java.lang.NullPointerException

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982)
	org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
	org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
Root Cause

java.lang.NullPointerException
	com.journaldev.spring.controller.HomeController.user(HomeController.java:38)
	sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	java.lang.reflect.Method.invoke(Method.java:498)

if(mutex ==null) mutex =""; //البرمجة الوقائية
		
synchronized(mutex) {
	System.out.println("synchronized block");
}

//استخدام فحوصات القيمة الفارغة
if(user!=null && user.getUserName() !=null) {
System.out.println("User Name: "+user.getUserName().toLowerCase());
}
if(user!=null && user.getUserName() !=null) {
	System.out.println("User ID: "+user.getUserId().toLowerCase());
}

public void foo(String s) {
    if(s.equals("Test")) {
	System.out.println("test");
    }
}

public void foo(String s) {
	if ("Test".equals(s)) {
		System.out.println("test");
	}
}

public int getArrayLength(Object[] array) {
	
	if(array == null) throw new IllegalArgumentException("array is null");
	
	return array.length;
}

String msg = (str == null) ? "" : str.substring(0, str.length()-1);

public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

Object mutex = null;

// يطبع قيمة فارغة
System.out.println(String.valueOf(mutex));

// سيُرمي java.lang.NullPointerException
System.out.println(mutex.toString());