java面试之finally与return

最新推荐文章于 2025-06-09 14:19:27 发布

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最新推荐文章于 2025-06-09 14:19:27 发布

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分类专栏： java

本文链接：https://blog.youkuaiyun.com/conjimmy/article/details/45910033

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本文通过多个示例详细解析了Java中finally块与return语句的交互作用，展示了不同情况下程序如何执行，以及finally块中return的影响。

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1.若finally前面有return，怎么样？

答： finally会在return前执行

例题一：

public classTest {

public static void main(String[] args) {
System.out.println(newTest().test());
}
staticint test()
{
int x = 1;
try
{
return x;
}
finally
{
++x;
}
}
}

返回1，说明是x先放到返回的填充的地方，然后再在finally加一，但对返回无影响，然后return 返回；

例题二：返回：2

public classTest {

public static void main(String[] args) {
System.out.println(newTest().test());
}
staticint test()
{
int x = 1;
try
{
return ++x;
}
finally
{
++x;
}
}

例题三：返回：1

public class MyTest {
static byte[] buffer = new byte[1024];
public static void main(String[] args) {
int n = -1;
try {
n = getString();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("result=" + n);
}

public static int getString() throws MyException {
FileInputStream fi = null;
int n = 0;
try {
boolean b = false;
fi = new FileInputStream(new File("./2.txt"));
n = fi.read(buffer);
n = 3;
return n;
} catch (FileNotFoundException e) {
e.printStackTrace();
return ++n;
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
n++;
if (fi != null) {
try {
fi.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return n;
}

}

例题四：报错，且返回：-1，

java.lang.NullPointerException
at com.jimmy.MyTest.getString(MyTest.java:58)
at com.jimmy.MyTest.main(MyTest.java:20)
result=-1

public class MyTest {
static byte[] buffer = new byte[1024];
public static void main(String[] args) {
int n = -1;
try {
n = getString();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("result=" + n);
}

public static int getString() throws MyException {
FileInputStream fi = null;
int n = 0;
try {
boolean b = false;
//fi = new FileInputStream(new File("./2.txt"));注意这里注销了，fi为空
n = fi.read(buffer);
n = 3;
return n;
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
return ++n;
} catch (IOException e) {
e.printStackTrace();
} finally {
n++;
try {
fi.close(); // 同样fi为空,但是这里没有NullPointerException的处理，所以往上抛到了，被getString（）的异常处理捕获
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return n; //这里没执行
}
}
例题五（reference：http://blog.youkuaiyun.com/hguisu/article/details/6155636）：返回：

i=2
i=1
testEx2, catch exception
testEx2, finally; return value=false
testEx1, finally; return value=false
testEx, finally; return value=false

public class MyExceptionTest {
public MyExceptionTest() {
}

boolean testEx() throws Exception {
boolean ret = true;
try {
ret = testEx1();
} catch (Exception e) {
System.out.println("testEx, catch exception");   //没有异常被抛出，这里不执行
ret = false;
throw e;
} finally {
System.out.println("testEx, finally; return value=" + ret);
return ret;
}
}

boolean testEx1() throws Exception {
boolean ret = true;
try {
ret = testEx2();
if (!ret) {
return false;
}
System.out.println("testEx1, at the end of try");
return ret;
} catch (Exception e) {
System.out.println("testEx1, catch exception"); //没有异常被抛出，这里不执行
ret = false;
throw e;
} finally {
System.out.println("testEx1, finally; return value=" + ret);
return ret;
}
}

boolean testEx2() throws Exception {
boolean ret = true;
try {
int b = 12;
int c;
for (int i = 2; i >= -2; i--) {
c = b / i;
System.out.println("i=" + i);
}
return true;
} catch (Exception e) {
System.out.println("testEx2, catch exception");
ret = false;
throw e; //这里相当于return，可是return前要运行finally ，可在finally里有return ，就提前返回了
} finally {
System.out.println("testEx2, finally; return value=" + ret);
return ret;
}
}

public static void main(String[] args) {
MyExceptionTest testException1 = new MyExceptionTest();
try {
testException1.testEx();
} catch (Exception e) {
e.printStackTrace();
}
}
}