为何无法调用抽象父类中继承的build()方法？

报错的核心原因是父类嵌套Builder的访问权限与链式调用的类型不匹配：

1. 抽象父类AbstractGetCommentExpectationTester的嵌套Builder类被定义为protected abstract static class，该权限仅允许同一个包内的类、父类的子类访问，测试类不在这个范围内。
2. 父类Builder的所有set方法返回的是父类Builder类型（AbstractGetCommentExpectationTester.Builder），当你在测试代码中链式调用setExpectedOwnerUsername("mickey")后，返回的实例类型变为父类Builder，而测试类无法访问这个protected类的build()方法，因此触发错误。

方案1：修改父类Builder的访问权限为public #

直接将父类中的嵌套Builder类权限改为public，让测试类可以正常访问其所有方法：

// 父类AbstractGetCommentExpectationTester中的Builder修改为public
public abstract static class Builder {
    protected final String serializedResponseBody;
    protected String expectedCommentText = "text";
    protected int expectedQuestionId = 1;
    protected int expectedOwnerId = 1;
    protected String expectedOwnerUsername = "user";

    protected Builder(MockHttpServletResponse response) throws UnsupportedEncodingException {
        this.serializedResponseBody = response.getContentAsString();
    }

    public Builder setExpectedOwnerUsername(String expectedOwnerUsername) {
        this.expectedOwnerUsername = expectedOwnerUsername;
        return this;
    }

    // 其余set方法和build方法保持不变
}

方案2：子类Builder重写set方法，使用协变返回类型 #

如果不想改变父类Builder的访问权限，可以在子类GetCommentExpectationTester.Builder中重写所有set方法，返回子类Builder类型，确保链式调用后始终保持子类Builder实例：

public static class Builder extends AbstractGetCommentExpectationTester.Builder {
    public Builder(MockHttpServletResponse response) throws UnsupportedEncodingException {
        super(response);
    }

    @Override
    public GetCommentExpectationTester build() {
        return new GetCommentExpectationTester(this);
    }

    // 重写所有set方法，返回子类Builder类型
    @Override
    public Builder setExpectedOwnerUsername(String expectedOwnerUsername) {
        super.setExpectedOwnerUsername(expectedOwnerUsername);
        return this;
    }

    @Override
    public Builder setExpectedOwnerId(int expectedOwnerId) {
        super.setExpectedOwnerId(expectedOwnerId);
        return this;
    }

    @Override
    public Builder setExpectedQuestionId(int expectedQuestionId) {
        super.setExpectedQuestionId(expectedQuestionId);
        return this;
    }

    @Override
    public Builder setExpectedCommentText(String expectedCommentText) {
        super.setExpectedCommentText(expectedCommentText);
        return this;
    }
}

内容的提问来源于stack exchange，提问作者Sergey Zolotarev