如何优雅的解决java.lang.NullPointerException 空指针的问题

背景

每次拿到对象在进行相关方法或属性的时候都要进行如下操作

先判断是否为null
if(obj!=null){
    //todo.......
}else{
 	 //todo
}

这样的代码，既繁琐，又影响阅读性，而且一不小心就会报如下错误信息

D:\Java\jdk1.8.0_202\bin\java.exe "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2018.3.5\lib\idea_rt.jar=55047:C:\Program Files\JetBrains\IntelliJ IDEA 2018.3.5\bin" -Dfile.encoding=UTF-8 -classpath 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com.hospital.hospital.service.impl.ImportServiceImpl
Exception in thread "main" java.lang.NullPointerException
	at com.hospital.hospital.service.impl.ImportServiceImpl.main(ImportServiceImpl.java:160)

Process finished with exit code 1

解决方案

怎么才能优雅的解决这个问题呢！
答案当然是有的，jdk1.8 的TOptional 为我们提供了优雅的解决方案，那么我们先看看源码

/**
     * Returns an {@code Optional} with the specified present non-null value.
     *
     * @param <T> the class of the value
     * @param value the value to be present, which must be non-null
     * @return an {@code Optional} with the value present
     * @throws NullPointerException if value is null
     */
    public static <T> Optional<T> of(T value) {
        return new Optional<>(value);
    }

    /**
     * Constructs an instance with the value present.
     *
     * @param value the non-null value to be present
     * @throws NullPointerException if value is null
     */
    private Optional(T value) {
        this.value = Objects.requireNonNull(value);
    }

  /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * </pre></blockquote>
     *
     * @param obj the object reference to check for nullity
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

顺着方法一步步走查，发现，如果T value是null，那么使用Optional.of(T value)就会手动抛出NullPointerException()。

所以，使用该方法格外注意，不能有null。

使用Optional.ofNullable，看源码

/**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     * @param <T> the class of the value
     * @param value the possibly-null value to describe
     * @return an {@code Optional} with a present value if the specified value
     * is non-null, otherwise an empty {@code Optional}
     */
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

    /**
     * Returns an empty {@code Optional} instance.  No value is present for this
     * Optional.
     *
     * @apiNote Though it may be tempting to do so, avoid testing if an object
     * is empty by comparing with {@code ==} against instances returned by
     * {@code Option.empty()}. There is no guarantee that it is a singleton.
     * Instead, use {@link #isPresent()}.
     *
     * @param <T> Type of the non-existent value
     * @return an empty {@code Optional}
     */
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }

    /**
     * Common instance for {@code empty()}.
     */
    private static final Optional<?> EMPTY = new Optional<>();

    /**
     * Constructs an empty instance.
     *
     * @implNote Generally only one empty instance, {@link Optional#EMPTY},
     * should exist per VM.
     */
    private Optional() {
        this.value = null;
    }

    /**
     * Return the value if present, otherwise return {@code other}.
     *
     * @param other the value to be returned if there is no value present, may
     * be null
     * @return the value, if present, otherwise {@code other}
     */
    public T orElse(T other) {
        return value != null ? value : other;
    }

这个方法可以，如果value为null，那么就会手动创建一个new Optional()；但是this.value = null。所以这个时候就跳出了手动抛空指针那个代码块。这个时候，我们就可以使用orElse(T other)。看源码，如果value == null，那么将取other的值

扩展

除了上面讲的ofNullable()，orElse()， 还有orElseGet(), isPresent(),map(),filter(),flatMap(),orElseThrow() 等等使用方法，有时间的可以深入研究一下。。。