Java 中的 NullPointerException

2024年9月10日 | 阅读 6 分钟

在本教程中，我们将学习 Java 中的空指针异常。空指针异常是运行时异常。Null 是一种特殊类型的值，可以赋给对象的引用。每当有人尝试使用具有 Null 值的引用时，就会引发 NullPointerException。

空指针异常的不同场景

观察以下可能引发 NullPointerException 的一些场景。

计算 Null 的大小或长度，就好像它是一个元素数组一样。

文件名： ThrowNullExcep.java

public class ThrowNullExcep
{
// main method
public static void main(String args[])
	{
		int arr[] = null; // array is assigned a null value
		System.out.println("The length of the array arr is: " + arr.length);
	}
}

输出

Exception in thread "main" java.lang.NullPointerException: Cannot read the array length because "" is null at ThrowNullExcep.main(ThrowNullExcep.java:7)

使用具有 Null 值的对象调用方法。

文件名： ThrowNullExcep1.java

public class ThrowNullExcep1
{
	public void foo()
	{
	    System.out.println("In the method foo.");
	}

	public static void main(String args[])
	{
		ThrowNullExcep1 obj = null; // assigning null value
		
		// invoking the method foo()
		obj.foo();
	}
}

输出

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "ThrowNullExcep1.foo()" because "" is null
	at ThrowNullExcep1.main(ThrowNullExcep1.java:13)

当您尝试同步 NULL 对象时。

文件名： ThrowNullExcep2.java

// A Java program that synchronizes over a NULL object.
import java.util.*;
import java.io.*;

// A Class that is required for sending the message
class Sendr
{
public void sendMethod(String mssg)
{
	System.out.println("Sending message: " + mssg );
	try
	{
		Thread.sleep(100);
	}
	catch (Exception exp)
	{
		System.out.println("Thread interrupted." + exp);
	}
	System.out.println("\n" + mssg + " is sent");
}
}

// A Class that is used to send messages with the help of threads Threads
class ThreadSend extends Thread
{
private String mssg;
Sendr sendr;

// Received a messge obj and the string
// mssge that has to be sent
ThreadSend(String mStr, Sendr obj)
{
	mssg = mStr;
	sendr = obj;
}

public void run()
{
	// Only a single thread is allowed to send a message
	// at a time.
	synchronized(sendr)
	{
		// synchronizing the send object
		sendr.sendMethod(mssg);
	}
}
}

// Driver class
public class ThrowNullExcep2
{
// main method
public static void main(String args[])
{
	Sendr sendObj = null;
	ThreadSend Sth1 = new ThreadSend( " Hello " , sendObj );
	ThreadSend Sth2 = new ThreadSend( " Bye Bye " , sendObj );

	// Starting the two threads of the ThreadedSend type
	Sth1.start();
	Sth2.start();

	// waiting for the threads to end
	try
	{
		Sth1.join();
		Sth2.join();
	}
	catch(Exception exp)
	{
		System.out.println("Interrupted : " + exp);
	}
}
}

输出

Exception in thread "Thread-0" Exception in thread "Thread-1" java.lang.NullPointerException: Cannot enter synchronized block because "this.sendr" is null
	at ThreadSend.run(ThrowNullExcep2.java:42)
java.lang.NullPointerException: Cannot enter synchronized block because "this.sendr" is null
	at ThreadSend.run(ThrowNullExcep2.java:42)

而不是抛出值，而是抛出 Null。

文件名： ThrowNullExcep3.java

// Modifying or accessing the fields of the Null object. 
public class ThrowExcep3
{
	int a;

    // main method
	public static void main(String args[])
	{
	    //  assigning a null value
		ThrowExcep3 obj = null;
		obj.a = 3;
	}
}

输出

Exception in thread "main" java.lang.NullPointerException: Cannot assign field "a" because "" is null
	at ThrowExcep3.main(ThrowExcep3.java:10)

Null 值的需求

null 是 Java 中使用的一种特殊值。它通常用于表示没有值被赋给引用变量。Null 值主要用于实现数据结构，如链表或树。它也用于单例模式。

避免 NullPointerException

为了避免 NullPointerException，在可以使用所有对象之前，应该确保正确初始化了所有对象。在声明引用变量时，在使用引用访问字段或方法之前，应验证是否未将 null 值赋给引用。

观察以下常见问题及其解决方案。

情况 1：将字符串与文字进行比较

一个常见的问题是将文字与 String 变量进行比较。文字可以是枚举元素或字符串。与其从 null 对象调用方法，不如考虑使用文字调用它。

文件名： NullPntrExcption.java

import java.io.*;
 
public class NullPntrExcption
{
// main method
public static void main (String[] args)
{
    // Initializing a String variable with a null value
    String pntr = null;

    // Checking if pntr.equals null or works fine.
    try
    {
        // The following line of code will raise the NullPointerException
        // It is because the pntr is null
        if (pntr.equals("JTP"))
        {
            System.out.print("String are the same.");
        }
        else
        {
            System.out.print("Strinng are not the same.");
        }
    }
    catch(NullPointerException e)
    {
        System.out.print("NullPointerException has been caught.");
    }
}
}

输出

NullPointerException has been caught.

现在，让我们看看如何避免它。

文件名： NullPntrExcption1.java

public class NullPntrExcption1
{
// main method
public static void main (String[] args)
{
    // Initializing a String variable with a null value
    String pntr = null;

    // Checking if pntr.equals null or works fine.
    try
    {
        // Now, the following line of code will not raise the NullPointerException
        // It is because the string literal is invoking the equals() method
        if ("JTP".equals(pntr))
        {
            System.out.print("String are the same.");
        }
        else
        {
            System.out.print("Strinng are not the same.");
        }
    }
    catch(NullPointerException e)
    {
        System.out.print("NullPointerException has been caught.");
    }
}
}

输出

NullPointerException has been caught.

情况 2：注意方法参数

必须先检查方法参数是否为 null 值，然后再执行方法。否则，很有可能会抛出 IllegalArgumentException，并向调用方法指示传递的参数有问题。

文件名： NullPntrExcption2.java

// A program for demonstrating that one must
// check the parameters are null or not before
// using them.
import java.io.*;

public class NullPntrExcption2
{
public static void main (String[] args)
{
    // String st is an empty string  and invoking method getLength()
    String st = "";
    try
    {
        System.out.println(getLength(st));
    }
    catch(IllegalArgumentException exp)
    {
        System.out.println("IllegalArgumentException has been caught.");
    }

    // String s set to a value and invoking method getLength()
    st = "JTP";
    try
    {
        System.out.println(getLength(st));
    }
    catch(IllegalArgumentException exp)
    {
        System.out.println("IllegalArgumentException has been caught.");
    }

    // Setting st with a value null and invoking method getLength()
    st = null;
    try
    {
        System.out.println(getLength(st));
    }
    catch(IllegalArgumentException exp)
    {
        System.out.println("IllegalArgumentException has been caught. " + exp);
    }
}

// method taht computes the length of a string st. 
// It throws and IllegalArgumentException, if st is null
public static int getLength(String st)
{
    if (st == null)
    {
        throw new IllegalArgumentException("The argument can never be null.");
    }
    return st.length();
}
}

输出

0
3
IllegalArgumentException has been caught. java.lang.IllegalArgumentException: The argument can never be null.

情况 3：使用三元运算符

也可以使用三元运算符来避免 NullPointerException。在三元运算符中，首先评估布尔表达式。如果表达式评估为 true，则返回 val1。否则，返回 val2。

文件名： NullPntrExcption3.java

// A program for demonstrating the fact that 
// NullPointerException can be avoided using the ternary operator.
import java.io.*;
 
public class NullPntrExcption3
{
    //  main method
    public static void main (String[] args)
    {
        // Initializing String variable with null value
        String st = null;
        String mssge = (st == null) ? "String is Null." : st.substring(0, 5);
        System.out.println(mssge);
 
        // Initializing the String variable with string literal
        st = "javaTpoint";
        mssge = (st == null) ? "" : st.substring(0, 10);
        System.out.println(mssge);
    }
}

输出

String is Null.
javaTpoint

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Java 中的 NullPointerException

空指针异常的不同场景

Null 值的需求

避免 NullPointerException

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