java - 将 null 值作为参数传递给方法时如何修复 NullPointerException

Question

这个问题已经有答案了:

How to handle null pointer and what is "null safe" way to code? [duplicate] (4 个回答)

已关闭 3 年前。

我的目标是使用自动生成测试来查找这些错误的工具来修复多个 Java 类中的错误。其中一个错误是 NullPointerException，它是由于两个属性被初始化为 null 并传递到对象中而引起的。同样，这是由该工具自动生成的，因此我的目标是修复类文件。

这是该工具生成的代码:

public void test1() throws Throwable {
        if (debug)
            System.out.format("%n%s%n", "ErrorTest0.test1");
        Point point0 = null;
        Point point1 = null;
        Line line2 = new Line(point0, point1); // Line 16
    }

这是导致问题的构造函数:

        public Line(Point p1, Point p2)
        {
                this.point1 = new Point(p1.x,p1.y); // Line 17
                this.point2 = new Point(p2.x,p2.y);
        }

以及 Point 的构造函数:

        public Point(double x, double y)
        {
                this.x = x;
                this.y = y;
        }

这是我从该工具收到的错误消息:

java.lang.NullPointerException
        at Line.<init>(Line.java:17)
        at ErrorTest0.test1(ErrorTest0.java:16)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
        at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
        at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
        at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
        at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
        at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
        at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
        at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
        at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
        at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
        at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
        at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
        at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
        at org.junit.runners.Suite.runChild(Suite.java:128)
        at org.junit.runners.Suite.runChild(Suite.java:27)
        at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
        at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
        at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
        at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
        at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
        at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
        at org.junit.runners.Suite.runChild(Suite.java:128)
        at org.junit.runners.Suite.runChild(Suite.java:27)
        at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
        at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
        at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
        at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
        at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
        at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
        at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
        at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
        at org.junit.runner.JUnitCore.runMain(JUnitCore.java:77)
        at org.junit.runner.JUnitCore.main(JUnitCore.java:36)

我猜我应该在构造函数中包含一些不允许 this 为空的内容？

Answer 1

您可以使用三种方法来更新线路方法。希望您的测试工具能够识别这些内容。

要求构造函数中的参数不为空。这是更现代的方法。

public Line(@NotNull Point p1, @NotNull Point p2) {

    this.point1 =  Objects.requireNonNull(p1);
    this.point2 =  Objects.requireNonNull(p2);
}

当参数设置为null时抛出异常。您的测试用例应该允许您预期该用例会出现异常。

public Line(Point p1, Point p2) throws IllegalArgumentException {

    if (p1 == null || p2 == null) {
        throw new IllegalArgumentException("point cannot be null");    
    } 

    this.point1 = p1;
    this.point2 = p2;
}

传入 null 时有一个合理的默认行为。什么是合理的完全取决于您的用例。

public Line(Point p1, Point p2) {

    if (p1 != null) {
        this.point1 = p1;
    } else {
        this.point1 = new Point(0.0, 0.0);
    }

    if (p2 != null) {  
        this.point2 = p2;
    } else {
        this.point2 = new Point(0.0, 0.0);
    }     
}