## Join 任何時候，只要在另一個Observable發射的數據定義的時間窗口內，這個Observable發射了一條數據，就結合兩個Observable發射的數據。  `Join`操作符結合兩個Observable發射的數據，基于時間窗口（你定義的針對每條數據特定的原則）選擇待集合的數據項。你將這些時間窗口實現為一些Observables，它們的生命周期從任何一條Observable發射的每一條數據開始。當這個定義時間窗口的Observable發射了一條數據或者完成時，與這條數據關聯的窗口也會關閉。只要這條數據的窗口是打開的，它將繼續結合其它Observable發射的任何數據項。你定義一個用于結合數據的函數。  很多ReactiveX實現還有一個類似的`GroupJoin`操作符。 Most ReactiveX implementations that have a Join operator also have a GroupJoin operator that is similar, except that the function you define to combine items emitted by the two Observables pairs individual items emitted by the source Observable not with an item from the second Observable, but with an Observable that emits items from the second Observable that fall in the same window.  The `join` operator takes four parameters: 1. the second Observable to combine with the source Observable 2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable 3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable 4. a function that accepts an item from the first Observable and an item from the second Observable and returns an item to be emitted by the Observable returned from join `join`默認不在任何特定的調度器上執行。 * Javadoc: [Join(Observable,Func1,Func1,Func2)](http://reactivex.io/RxJava/javadoc/rx/Observable.html#join(rx.Observable,%20rx.functions.Func1,%20rx.functions.Func1,%20rx.functions.Func2))  The `groupJoin` operator takes four parameters: 1. the second Observable to combine with the source Observable 2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable 3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable 4. a function that accepts an item from the first Observable and an Observable that emits items from the second Observable and returns an item to be emitted by the Observable returned from groupJoin `groupJoin `默認不在任何特定的調度器上執行。 * Javadoc: [groupJoin(Observable,Func1,Func1,Func2)](http://reactivex.io/RxJava/javadoc/rx/Observable.html#groupJoin(rx.Observable,%20rx.functions.Func1,%20rx.functions.Func1,%20rx.functions.Func2))  可選的`StringObservable`類中也有一個`join`操作符。它將一個發射字符串序列的Observable轉換為一個發射單個字符串的Observable，`join`操作符使用指定的定界符將全部單獨的字符串連接起來。