if 语句后面可以跟 else 语句，当 if 语句的布尔表达式值为 false 时，else 语句块会被执行。
if...else - 语法
if(Boolean_expression) { //当布尔表达式为true时执行 }else { //当布尔表达式为false时执行 }
if...else - 流程图
if...else - 示例
public class Test {
public static void main(String args[]) {
int x=30;
</span><span class="kwd">if</span><span class="pun">(</span><span class="pln"> x </span><span class="pun"><</span><span class="pln"> </span><span class="lit">20</span><span class="pln"> </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="kwd">print</span><span class="pun">(</span><span class="str">"This is if statement"</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span><span class="kwd">else</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="kwd">print</span><span class="pun">(</span><span class="str">"This is else statement"</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
}
}
这将产生以下输出-
This is else statement
if ... else if ... else - 语句
if语句后可以跟可选的 else if ... else 语句，这对于使用单个if ... else if语句测试各种条件非常有用。
if(Boolean_expression 1) { //当布尔表达式 1 为true时执行 }else if(Boolean_expression 2) { //当布尔表达式 2 为true时执行 }else if(Boolean_expression 3) { //当布尔表达式 3 为true时执行 }else { //当上述条件均不成立时执行。 }
if ... else if ... else - 示例
public class Test {
public static void main(String args[]) {
int x=30;
</span><span class="kwd">if</span><span class="pun">(</span><span class="pln"> x </span><span class="pun">==</span><span class="pln"> </span><span class="lit">10</span><span class="pln"> </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="kwd">print</span><span class="pun">(</span><span class="str">"Value of X is 10"</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span><span class="kwd">else</span><span class="pln"> </span><span class="kwd">if</span><span class="pun">(</span><span class="pln"> x </span><span class="pun">==</span><span class="pln"> </span><span class="lit">20</span><span class="pln"> </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="kwd">print</span><span class="pun">(</span><span class="str">"Value of X is 20"</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span><span class="kwd">else</span><span class="pln"> </span><span class="kwd">if</span><span class="pun">(</span><span class="pln"> x </span><span class="pun">==</span><span class="pln"> </span><span class="lit">30</span><span class="pln"> </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="kwd">print</span><span class="pun">(</span><span class="str">"Value of X is 30"</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span><span class="kwd">else</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="kwd">print</span><span class="pun">(</span><span class="str">"This is else statement"</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
}
}
这将产生以下输出-
Value of X is 30
