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/*
 * Copyright (C) 2012 The Dagger Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package dagger;

/**
 * A handle to a lazily-computed value. Each {@code Lazy} computes its value on
 * the first call to {@link #get()} and remembers that same value for all
 * subsequent calls to {@code get()}.
 *
 * 

All implementations are expected to be thread-safe and compute their value at most once. * *

Example

* The differences between direct injection, provider * injection and lazy injection are best demonstrated * with an example. Start with a module that computes a different integer for * each use:

 *   {@literal @Module}
 *   final class CounterModule {
 *     int next = 100;
 *
 *     {@literal @Provides} Integer provideInteger() {
 *       System.out.println("computing...");
 *       return next++;
 *     }
 *   }
 * 
* *

Direct Injection

* This class injects that integer and prints it 3 times:

 *   final class DirectCounter {
 *     {@literal @Inject} Integer value;
 *
 *     void print() {
 *       System.out.println("printing...");
 *       System.out.println(value);
 *       System.out.println(value);
 *       System.out.println(value);
 *     }
 *   }
 * 
* Injecting a {@code DirectCounter} and invoking {@code print()} reveals that * the value is computed before it is required:

 *   computing...
 *   printing...
 *   100
 *   100
 *   100
 * 
* *

Provider Injection

* This class injects a {@linkplain javax.inject.Provider provider} for the * integer. It calls {@code Provider.get()} 3 times and prints each result: *

 *   final class ProviderCounter {
 *     {@literal @Inject Provider provider;}
 *
 *     void print() {
 *       System.out.println("printing...");
 *       System.out.println(provider.get());
 *       System.out.println(provider.get());
 *       System.out.println(provider.get());
 *     }
 *   }
 * 
* Injecting a {@code ProviderCounter} and invoking {@code print()} shows that * a new value is computed each time {@code Provider.get()} is used:

 *   printing...
 *   computing...
 *   100
 *   computing...
 *   101
 *   computing...
 *   102
 * 
* *

Lazy Injection

* This class injects a {@code Lazy} for the integer. Like the provider above, * it calls {@code Lazy.get()} 3 times and prints each result:

 *   final class LazyCounter {
 *     {@literal @Inject Lazy lazy;}
 *
 *     void print() {
 *       System.out.println("printing...");
 *       System.out.println(lazy.get());
 *       System.out.println(lazy.get());
 *       System.out.println(lazy.get());
 *     }
 *   }
 * 
* Injecting a {@code LazyCounter} and invoking {@code print()} shows that a new * value is computed immediately before it is needed. The same value is returned * for all subsequent uses:

 *   printing...
 *   computing...
 *   100
 *   100
 *   100
 * 
* *

Lazy != Singleton

* Note that each injected {@code Lazy} is independent, and remembers its value * in isolation of other {@code Lazy} instances. In this example, two {@code * LazyCounter} objects are created and {@code print()} is called on each: *

 *   final class LazyCounters {
 *     {@literal @Inject} LazyCounter counter1;
 *     {@literal @Inject} LazyCounter counter2;
 *
 *     void print() {
 *       counter1.print();
 *       counter2.print();
 *     }
 *   }
 * 
* The output demonstrates that each {@code Lazy} works independently: *

 *   printing...
 *   computing...
 *   100
 *   100
 *   100
 *   printing...
 *   computing...
 *   101
 *   101
 *   101
 * 
* Use {@link javax.inject.Singleton @Singleton} to share one instance among all * clients, and {@code Lazy} for lazy computation in a single client. */ public interface Lazy { /** * Return the underlying value, computing the value if necessary. All calls to * the same {@code Lazy} instance will return the same result. */ T get(); }




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