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[英]Java forcing default return statement from method with enum switch with all enum values covered
[英]How do I compare the value of an Enum with all possible values of the Enum in an Enum method and avoid missing return statement?
我只是在學習Java枚舉。 當我在下面運行代碼時,我得到一個錯誤,下面也會重現。 基本上,我的問題是:當我在枚舉中定義一個方法時,我想在該方法中檢查枚舉的值,以便可以基於該值執行某些操作,如何執行此檢查? 下面,我有一個帶有三個可能值的Enum,在方法getNext
,我有三個if語句,將這個Enum的值與三個可能值中的每一個進行比較。 但是我仍然收到一條錯誤消息,說存在一條沒有回報的道路。
package enumerations;
enum TrafficLightColor2 {
RED(12), GREEN(10), YELLOW(2);
private int waitTime;
TrafficLightColor2(int waitTime) {
this.waitTime = waitTime;
}
int getWaitTime() {
return waitTime;
}
TrafficLightColor2 getNext() {
if (this.equals(TrafficLightColor2.GREEN)) {
return TrafficLightColor2.YELLOW;
}
if (this.equals(TrafficLightColor2.YELLOW)) {
return TrafficLightColor2.RED;
}
if (this.equals(TrafficLightColor2.RED)) {
return TrafficLightColor2.GREEN;
}
}
}
// A computerized traffic light.
class TrafficLightSimulator2 implements Runnable {
private Thread thrd; // holds the thread that runs the simulation
private TrafficLightColor2 tlc; // holds the traffic light color
boolean stop = false; // set to true to stop the simulation
boolean changed = false; // true when the light has changed
TrafficLightSimulator2(TrafficLightColor2 init) {
tlc = init;
thrd = new Thread(this);
thrd.start();
}
TrafficLightSimulator2() {
tlc = TrafficLightColor2.RED;
thrd = new Thread(this);
thrd.start();
}
// Start up the light.
public void run() {
while (!stop) {
try {
Thread.sleep(tlc.getWaitTime());
} catch (InterruptedException exc) {
System.out.println(exc);
}
changeColor();
}
}
// Change color.
synchronized void changeColor() {
tlc = tlc.getNext();
changed = true;
notify(); // signal that the light has changed
}
// Wait until a light change occurs.
synchronized void waitForChange() {
try {
while (!changed)
wait(); // wait for light to change
changed = false;
} catch (InterruptedException exc) {
System.out.println(exc);
}
}
// Return current color.
synchronized TrafficLightColor2 getColor() {
return tlc;
}
// Stop the traffic light.
synchronized void cancel() {
stop = true;
}
}
class TrafficLightDemo2 {
public static void main(String args[]) {
TrafficLightSimulator tl =
new TrafficLightSimulator(TrafficLightColor.GREEN);
for (int i = 0; i < 9; i++) {
System.out.println(tl.getColor());
tl.waitForChange();
}
tl.cancel();
}
}
我得到錯誤
$ javac enumerations/TrafficLightDemo2.java
enumerations/TrafficLightDemo2.java:26: error: missing return statement
}
^
1 error
TrafficLightColor2 getNext() {
if (this.equals(TrafficLightColor2.GREEN)) {
return TrafficLightColor2.YELLOW;
}
if (this.equals(TrafficLightColor2.YELLOW)) {
return TrafficLightColor2.RED;
}
if (this.equals(TrafficLightColor2.RED)) {
return TrafficLightColor2.GREEN;
}
}
如果所有3個if
均為false,則此方法不返回值。
在處添加return或更好地拋出錯誤,例如
throw new IllegalArgumentException("Unsupported enum")
在枚舉類中使用實例字段的優點是,您可以輕松地將實現詳細信息與獨立於API的常量相關聯。 換句話說,您可以輕松地將數據與枚舉常量相關聯,這樣就可以接受一種優雅的解決方案,例如在您需要添加新的枚舉常量的情況下,您並不會永遠結婚。
因此,您可以在履行以下相同合同的同時極大地簡化實現:
enum TrafficLightColor2 {
RED(2, 12),
GREEN(0, 10),
YELLOW(1, 2);
private int order; // implementation detail; non-exported
private int waitTime;
TrafficLightColor2(int ord, int waitTime) {
this.order = ord;
this.waitTime = waitTime;
}
int getWaitTime() {
return waitTime;
}
TrafficLightColor2 getNext() {
final int nextColor = (this.order + 1) % 3; // magic numbers introduce fragility
return Arrays.stream(TrafficLight2.values())
.filter(e -> e.order == nextColor)
.findAny()
.get();
}
}
這個版本對您的原始實現有一些好處:易於維護,因為如果添加了枚舉常量,則編譯器將強制您添加訂單值。 在原始版本中,如果您在添加常量后忘記修改if-else-block,則程序將繼續運行,但不會提供正確的行為。 而且由於order
的實現是隱藏的,您可以隨時將其刪除或將其更改為其他實現,而不會影響API的正確性。
您是否考慮過將下一個狀態與聲明的值一起包含?
public enum TrafficLightColor2 {
RED(12, "GREEN"), GREEN(10, "YELLOW"), YELLOW(2, "RED");
int waitTime;
String nextState;
Configurations(int waitTime, String nextState) {
this.waitTime = waitTime;
this.nextState = nextState;
}
public int getWaitTime() {
return waitTime;
}
public String getNextState() {
return nextState;
}
}
有了這個,你可以得到下一個狀態
TrafficLightColor2 trafficLightColor = TrafficLightColor2.GREEN;
System.out.println(TrafficLightColor2.valueOf(trafficLightColor.getNextState()));
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