Guava中的Optional实现

Optional实现的业务代码：

ackage com.google.common.base;import com.google.common.annotations.Beta;import com.google.common.annotations.GwtCompatible;import com.google.common.base.Absent;import com.google.common.base.AbstractIterator;import com.google.common.base.Function;import com.google.common.base.Preconditions;import com.google.common.base.Present;import com.google.common.base.Supplier;import java.io.Serializable;import java.util.Iterator;import java.util.Set;import javax.annotation.Nullable;@Beta@GwtCompatible(    serializable = true)public abstract class Optional<T> implements Serializable {    private static final long serialVersionUID = 0L;    public static <T> Optional<T> absent() {        return Absent.INSTANCE;    }    public static <T> Optional<T> of(T reference) {        return new Present(Preconditions.checkNotNull(reference));    }    public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {        return (Optional)(nullableReference == null?absent():new Present(nullableReference));    }    Optional() {    }    public abstract boolean isPresent();    public abstract T get();    public abstract T or(T var1);    public abstract Optional<T> or(Optional<? extends T> var1);    public abstract T or(Supplier<? extends T> var1);    @Nullable    public abstract T orNull();    public abstract Set<T> asSet();    public abstract <V> Optional<V> transform(Function<? super T, V> var1);    public abstract boolean equals(@Nullable Object var1);    public abstract int hashCode();    public abstract String toString();    public static <T> Iterable<T> presentInstances(final Iterable<Optional<T>> optionals) {        Preconditions.checkNotNull(optionals);        return new Iterable() {            public Iterator<T> iterator() {                return new AbstractIterator() {                    private final Iterator<Optional<T>> iterator = (Iterator)Preconditions.checkNotNull(optionals.iterator());                    protected T computeNext() {                        while(true) {                            if(this.iterator.hasNext()) {                                Optional optional = (Optional)this.iterator.next();                                if(!optional.isPresent()) {                                    continue;                                }                                return optional.get();                            }                            return this.endOfData();                        }                    }                };            }        };    }}

package com.google.common.base;import com.google.common.annotations.GwtCompatible;import com.google.common.base.Function;import com.google.common.base.Optional;import com.google.common.base.Preconditions;import com.google.common.base.Supplier;import java.util.Collections;import java.util.Set;import javax.annotation.Nullable;@GwtCompatiblefinal class Absent extends Optional<Object> {    static final Absent INSTANCE = new Absent();    private static final long serialVersionUID = 0L;    Absent() {    }    public boolean isPresent() {        return false;    }    public Object get() {        throw new IllegalStateException("value is absent");    }    public Object or(Object defaultValue) {        return Preconditions.checkNotNull(defaultValue, "use orNull() instead of or(null)");    }    public Optional<Object> or(Optional<?> secondChoice) {        return (Optional)Preconditions.checkNotNull(secondChoice);    }    public Object or(Supplier<?> supplier) {        return Preconditions.checkNotNull(supplier.get(), "use orNull() instead of a Supplier that returns null");    }    @Nullable    public Object orNull() {        return null;    }    public Set<Object> asSet() {        return Collections.emptySet();    }    public <V> Optional<V> transform(Function<Object, V> function) {        Preconditions.checkNotNull(function);        return Optional.absent();    }    public boolean equals(@Nullable Object object) {        return object == this;    }    public int hashCode() {        return 1502476572;    }    public String toString() {        return "Optional.absent()";    }    private Object readResolve() {        return INSTANCE;    }}

package com.google.common.base;import com.google.common.annotations.GwtCompatible;import com.google.common.base.Function;import com.google.common.base.Optional;import com.google.common.base.Preconditions;import com.google.common.base.Supplier;import java.util.Collections;import java.util.Set;import javax.annotation.Nullable;@GwtCompatiblefinal class Present<T> extends Optional<T> {    private final T reference;    private static final long serialVersionUID = 0L;    Present(T reference) {        this.reference = reference;    }    public boolean isPresent() {        return true;    }    public T get() {        return this.reference;    }    public T or(T defaultValue) {        Preconditions.checkNotNull(defaultValue, "use orNull() instead of or(null)");        return this.reference;    }    public Optional<T> or(Optional<? extends T> secondChoice) {        Preconditions.checkNotNull(secondChoice);        return this;    }    public T or(Supplier<? extends T> supplier) {        Preconditions.checkNotNull(supplier);        return this.reference;    }    public T orNull() {        return this.reference;    }    public Set<T> asSet() {        return Collections.singleton(this.reference);    }    public <V> Optional<V> transform(Function<? super T, V> function) {        return new Present(Preconditions.checkNotNull(function.apply(this.reference), "Transformation function cannot return null."));    }    public boolean equals(@Nullable Object object) {        if(object instanceof Present) {            Present other = (Present)object;            return this.reference.equals(other.reference);        } else {            return false;        }    }    public int hashCode() {        return 1502476572 + this.reference.hashCode();    }    public String toString() {        return "Optional.of(" + this.reference + ")";    }}

调用的Optional入口的方法：

    private static Optional<Connection> getConnection() {        Connection connection = null;        try {            connection = ComboPool.getConnection();        } catch (SQLException e) {            log.error("Get connection from pool error.", e);        }        return Optional.fromNullable(connection);    }

根据调用方法方法返回的Optional，直接调用

 while (!connectionOptional.isPresent()) {                log.warn("Can't get connection from database, wait for one minute, try again.");                Thread.sleep(60000);                connectionOptional = getConnection();            }

可以判断是获取的发返回值是否为null。
这是一个可以为null的容器对象。如果值存在则isPresent()方法会返回true，调用get()方法会返回该对象。