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