[Java] java8新特性之Optional的深入解析

public final class optional<t> {}

public class java8optionaltest {
 list<string> stringlist = null;
 icar car = new weilaicar();
}
 
public class weilaicar implements icar {
 integer wheels = new integer(4);
｝

• public final class optional<t> {
  
• public final class optionaldouble {
  
• public final class optionalint {
  
• public final class optionallong {
  

@functionalinterface
predicate\consumer\supplier三个接口都是函数式接口

private optional() {
 this.value = null;
}

public static <t> optional<t> of(t value) {
 return new optional<>(value);
}
public static <t> optional<t> ofnullable(t value) {
 return value == null ? empty() : of(value);
}
public static<t> optional<t> empty() {
 @suppresswarnings("unchecked")
 optional<t> t = (optional<t>) empty;
 return t;
}

public boolean ispresent() {
 return value != null;
}
public void ifpresent(consumer<? super t> consumer) {
 if (value != null)
 consumer.accept(value);
}

java8optionaltest test = new java8optionaltest();
optional<java8optionaltest> optional = optional.of(test);
 
pringtest(optional.ispresent());
//true
optional.ifpresent( a -> pringtest(a.getcar().getclass().getname()));
//com.ts.util.optional.weilaicar
optional.ifpresent( a -> optional.ofnullable(a.getstringlist()).ifpresent(b -> pringtest("stringlist:" + (b == null))));
//第一级的ifpresent是存在test对象，所以执行了lambda表达式，而第二级的ifpresent的stringlist是null，所以没有执行表达式
optional.ifpresent( a -> optional.ofnullable(a.getcar()).ifpresent(b -> pringtest("car:" + (b == null))));
//car:false
//第二级ifpresent的car对象是存在的，所以第二级的表达式执行了

• map方法的参数是个当包含的对象不为null时才执行的lambda表达式，返回该表达式执行结果的封装optional对象，同理支持链式调用，逐层深入和递归递进很像；
  
• flatmap区别在于lambda表达式的返回结果必须主动包裹optinoal，否则报错
  

public<u> optional<u> map(function<? super t, ? extends u> mapper) {
 objects.requirenonnull(mapper);
 if (!ispresent())
  return empty();
 else {
  return optional.ofnullable(mapper.apply(value));
 }
}
public<u> optional<u> flatmap(function<? super t, optional<u>> mapper) {
 objects.requirenonnull(mapper);
 if (!ispresent())
  return empty();
 else {
  return objects.requirenonnull(mapper.apply(value));
 }
}

java8optionaltest test = new java8optionaltest();
optional<java8optionaltest> optional = optional.of(test);
 
optional opt1 = optional.map( a -> a.getcar());
pringtest(opt1.get());
//com.ts.util.optional.weilaicar@5d6f64b1
int wheel = 0;//传统null判断写法
if(test != null){
 if(test.getcar() != null){//实际业务里面层级也许会超过3层
  wheel = test.getcar().getwheelcount();
 }
}
pringtest("传统:"+wheel);
//传统:4
optional opt2 = optional.map( a -> a.getcar()).map(b -> b.getwheelcount());//optional支持下的写法
pringtest("optinal:"+opt2.get());
//optinal:4
optional opt3 = optional.map( a -> a.getstringlist()).map(b -> b.size());
pringtest(opt3);
//optional.empty
 
optional opt4 = optional.flatmap(a -> optional.of(a.getcar()));//主动包裹optional对象
pringtest(opt4);
//optional[com.ts.util.optional.weilaicar@5d6f64b1]
optional opt5 = optional.flatmap(a -> optional.of(a.getcar())).flatmap(b -> optional.ofnullable(b.getwheelcount()));
pringtest(opt5);
//optional[4]

public optional<t> filter(predicate<? super t> predicate) {
 objects.requirenonnull(predicate);
 if (!ispresent())
  return this;
 else
  return predicate.test(value) ? this : empty();
}

java8optionaltest test = new java8optionaltest();
 
optional<java8optionaltest> optional = optional.of(test);
 
optional result = optional.filter( a -> a.getcar() != null).filter( b -> b.getclass().getname() != null);
pringtest(result.ispresent()? result.get().getclass().getname(): result.ispresent());
//com.ts.util.java8optionaltest
optional result1 = optional.filter( a -> a.getstringlist() != null);
pringtest(result1.get());
//java.util.nosuchelementexception: no value present

• orelse 当optional内对象为null就返回这个参数，比较像很多默认值设置；
  
• orelseget 基本同orelse，区别是传入参数支持lambda表达式，返回的就是表达式执行结果；
  
• orelsethrow 也是传入lambda表达式，但是表达式是抛出异常
  

public t orelse(t other) {
 return value != null ? value : other;
}
 
public t orelseget(supplier<? extends t> other) {
 return value != null ? value : other.get();
}
 
public <x extends throwable> t orelsethrow(supplier<? extends x> exceptionsupplier) throws x {
 if (value != null) {
  return value;
 } else {
  throw exceptionsupplier.get();
 }
}

java8optionaltest one = null;
java8optionaltest test = new java8optionaltest();
optional<java8optionaltest> optional = optional.ofnullable(one);
pringtest(optional);
//optional.empty
pringtest(optional.orelse(test));
//com.ts.util.java8optionaltest@5197848c
pringtest(optional.orelseget(() -> new java8optionaltest()));
//com.ts.util.java8optionaltest@5d6f64b1
pringtest(optional.orelsethrow(() -> new runtimeexception("orelsethrow")));
//java.lang.runtimeexception: orelsethrow