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https://docs.oracle.com/javase/tutorial/java/nutsandbolts/if.html

# if/else
顧名思義，if 就是如果...，就...

 if (boolean expression) // statement

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範例:

int a = 1;
if (a == 1)
    System.out.print("hello world");

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前篇說過, Block也算是一個statement.

int a = 1;
if (a == 1) {
    System.out.print("hello world");
    System.out.print("hello world!!!");
}

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如果要在 if 後面在新增更多種情況可以用 else if

int a = 5487;
if (a > 10000) {
    System.out.print("too expensive");
} else if(a > 5000) {
    System.out.print("deal"); // 會執行這行
}

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if或是if else都不成立, 可以接 else

int a = 3000;
if (a > 10000) {
    System.out.print("too expensive");
} else if (a > 5000) {
    System.out.print("deal"); 
} else {
    System.out.print("cheapest is the dearest"); // 會執行這行
}

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不建議因為Block內只有一個敘述就省略大括號。

int a = 1;
if (a == 1)
    System.out.print("Hello World");
System.out.println("End of this section");

這樣寫比較好懂。

int a = 1;
if (a == 1) {
    System.out.print("Hello World");
}
System.out.println("End of this section");

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不能在非block的敘述內宣告變數。

if (a > 10000)
    int i = 1; // not allowed