为什么newInstance()返回null? [英] Why does newInstance() return null?

问题描述

根据 JavaDocs ，java.lang.Class的newInstance()方法无意返回null. 但是我的代码似乎证明是相反的.为什么?

According to the JavaDocs, the newInstance() Method of java.lang.Class is not intented to return null. But my code seems to prove the opposite. Why?

public Assessment createAssessment() {
    Class<? extends Assessment> assessmentClass = (Class<? extends Assessment>) assessmentClassDataTable.getRowData();
    try {
        System.out.println("ASSESSMENTCLASS " + assessmentClass);
        // -> 'ASSESSMENTCLASS class my.model.ManualSelectAssessment'
        Assessment a = assessmentClass.newInstance();
        System.out.println("ASSESSMENT " + a);
        // -> 'ASSESSMENT null'
        return a;
    } catch (Exception e) {
        Application.handleError(e);
    }
    return null;
}

它返回null.

推荐答案

newInstance()从不返回null.但是newInstance().toString()可以返回"null"

newInstance() never returns null. However newInstance().toString() can return "null"

注意:使用newInstance()的一个陷阱是它可以抛出CheckedException！

Note: One gotcha with newInstance() is that it can throw a CheckedException !

public Main() throws IOException {
    throw new IOException();
}

public static void main(String[] args) throws InstantiationException, IllegalAccessException {
    Main.class.newInstance(); // throws IOException silently.
}

即使IOException是检查异常，编译器也不认为newInstance()会抛出此检查异常.如果您试图抓住它，编译器会抱怨它不能被抛出！

Even though IOException is a check exception, the compiler has not idea the newInstance() will throw this checked exception. If you try to catch it the compiler will complain it cannot be thrown !!

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