消灭 Java 代码的“坏味道”

导读

私欲日生，如地上尘，一日不扫，便又有一层。着实用功，便见道无终穷，愈探愈深，必使精白无一毫不彻方可。

Map<String, String> map =...;
for(String key : map.keySet()){
 String value = map.get(key);
...
}

正例：

Map<String, String> map =...;
for(Map.Entry<String, String> entry : map.entrySet()){
 String key = entry.getKey();
 String value = entry.getValue();
...
}

if(collection.size()==0){
...
}

if(collection.isEmpty()){
...
}

List<String> list =newArrayList<>();
list.add("Hello");
list.add("World");
if(list.containsAll(list)){// 无意义,总是返回true
...
}
list.removeAll(list);// 性能差, 直接使用clear()

int[] arr =newint[]{1,2,3};
List<Integer> list =newArrayList<>();
for(int i : arr){
 list.add(i);
}

int[] arr =newint[]{1,2,3};
List<Integer> list =newArrayList<>(arr.length);
for(int i : arr){
 list.add(i);
}

String s ="";
for(int i =0; i <10; i++){
 s += i;
}

String a ="a";
String b ="b";
String c ="c";
String s = a + b + c;// 没问题，java编译器会进行优化
StringBuilder sb =newStringBuilder();
for(int i =0; i <10; i++){
 sb.append(i);// 循环中，java编译器无法进行优化，所以要手动使用StringBuilder
}

List 的随机访问

// 调用别人的服务获取到list
List<Integer> list = otherService.getList();
if(list instanceofRandomAccess){
// 内部数组实现，可以随机访问
 System.out.println(list.get(list.size()-1));
}else{
// 内部可能是链表实现，随机访问效率低
}

频繁调用 Collection.contains 方法请使用 Set

ArrayList<Integer> list = otherService.getList();
for(int i =0; i <= Integer.MAX_VALUE; i++){
// 时间复杂度O(n)
 list.contains(i);
}

ArrayList<Integer> list = otherService.getList();
Set<Integer> set =newHashSet(list);
for(int i =0; i <= Integer.MAX_VALUE; i++){
// 时间复杂度O(1)
 set.contains(i);
}

让代码更优雅

长整型常量后添加大写 L

long value = 1l;
long max = Math.max(1L,5);

long value = 1L;
long max = Math.max(1L, 5L);

不要使用魔法值

for(int i =0; i <100; i++){
...
}
if(a ==100){
...
}

privatestaticfinalint MAX_COUNT =100;
for(int i =0; i < MAX_COUNT; i++){
...
}
if(count == MAX_COUNT){
...
}

privatestatic Map<String, Integer> map =newHashMap<String, Integer>(){
{
put("a",1);
put("b",2);
}
};

privatestatic List<String> list =newArrayList<String>(){
{
add("a");
add("b");
}
};

privatestatic Map<String, Integer> map =newHashMap<>();
static{
 map.put("a",1);
 map.put("b",2);
};

privatestatic List<String> list =newArrayList<>();
static{
 list.add("a");
 list.add("b");
};

建议使用 try-with-resources 语句

privatevoidhandle(String fileName){
 BufferedReader reader = null;
try{
 String line;
 reader =newBufferedReader(newFileReader(fileName));
while((line = reader.readLine())!= null){
...
}
}catch(Exception e){
...
}finally{
if(reader != null){
try{
 reader.close();
}catch(IOException e){
...
}
}
}
}

privatevoidhandle(String fileName){
try(BufferedReader reader =newBufferedReader(newFileReader(fileName))){
 String line;
while((line = reader.readLine())!= null){
...
}
}catch(Exception e){
...
}
}

删除未使用的私有方法和字段

publicclassDoubleDemo1{
privateint unusedField =100;
privatevoidunusedMethod(){
...
}
publicintsum(int a,int b){
return a + b;
}
}

publicclassDoubleDemo1{
publicintsum(int a,int b){
return a + b;
}
}

删除未使用的局部变量

publicintsum(int a,int b){
int c =100;
return a + b;
}

publicintsum(int a,int b){
return a + b;
}

删除未使用的方法参数

publicintsum(int a,int b,int c){
return a + b;
}

publicintsum(int a,int b){
return a + b;
}

删除表达式的多余括号

return(x);
return(x +2);
int x =(y *3)+1;
int m =(n *4+2);

return x;
return x +2;
int x = y *3+1;
int m = n *4+2;

工具类应该屏蔽构造函数

publicclassMathUtils{
publicstaticfinaldouble PI =3.1415926D;
publicstaticintsum(int a,int b){
return a + b;
}
}

publicclassMathUtils{
publicstaticfinaldouble PI =3.1415926D;
privateMathUtils(){}
publicstaticintsum(int a,int b){
return a + b;
}
}

删除多余的异常捕获并抛出

privatestatic String readFile(String fileName)throws IOException {
try(BufferedReader reader =newBufferedReader(newFileReader(fileName))){
 String line;
 StringBuilder builder =newStringBuilder();
while((line = reader.readLine())!= null){
 builder.append(line);
}
return builder.toString();
}catch(Exception e){
throw e;
}
}

privatestatic String readFile(String fileName)throws IOException {
try(BufferedReader reader =newBufferedReader(newFileReader(fileName))){
 String line;
 StringBuilder builder =newStringBuilder();
while((line = reader.readLine())!= null){
 builder.append(line);
}
return builder.toString();
}
}

公有静态常量应该通过类访问

publicclassUser{
publicstaticfinal String CONST_NAME ="name";
...
}

User user =newUser();
String nameKey = user.CONST_NAME;

publicclassUser{
publicstaticfinal String CONST_NAME ="name";
...
}

String nameKey = User.CONST_NAME;

不要用NullPointerException判断空

public String getUserName(User user){
try{
return user.getName();
}catch(NullPointerException e){
return null;
}
}

正例：

public String getUserName(User user){
if(Objects.isNull(user)){
return null;
}
return user.getName();
}

使用String.valueOf(value)代替""+value

int i =1;
String s =""+ i;

int i =1;
String s = String.valueOf(i);

过时代码添加 @Deprecated 注解

/**
 * 保存
 *
 * @deprecated 此方法效率较低，请使用{@link newSave()}方法替换它
 */
@Deprecated
publicvoidsave(){
// do something
}

让代码远离 bug

禁止使用构造方法 BigDecimal(double)

BigDecimal value =newBigDecimal(0.1D);// 0.100000000000000005551115...

BigDecimal value = BigDecimal.valueOf(0.1D);;// 0.1

返回空数组和空集合而不是 null

publicstatic Result[]getResults(){
return null;
}

publicstatic List<Result>getResultList(){
return null;
}

publicstatic Map<String, Result>getResultMap(){
return null;
}

publicstaticvoidmain(String[] args){
 Result[] results =getResults();
if(results != null){
for(Result result : results){
...
}
}

 List<Result> resultList =getResultList();
if(resultList != null){
for(Result result : resultList){
...
}
}

 Map<String, Result> resultMap =getResultMap();
if(resultMap != null){
for(Map.Entry<String, Result> resultEntry : resultMap){
...
}
}
}

publicstatic Result[]getResults(){
returnnewResult[0];
}

publicstatic List<Result>getResultList(){
return Collections.emptyList();
}

publicstatic Map<String, Result>getResultMap(){
return Collections.emptyMap();
}

publicstaticvoidmain(String[] args){
 Result[] results =getResults();
for(Result result : results){
...
}

 List<Result> resultList =getResultList();
for(Result result : resultList){
...
}

 Map<String, Result> resultMap =getResultMap();
for(Map.Entry<String, Result> resultEntry : resultMap){
...
}
}

publicvoidisFinished(OrderStatus status){
return status.equals(OrderStatus.FINISHED);// 可能抛空指针异常
}

publicvoidisFinished(OrderStatus status){
return OrderStatus.FINISHED.equals(status);
}

publicvoidisFinished(OrderStatus status){
return Objects.equals(status, OrderStatus.FINISHED);
}

publicenum UserStatus { DISABLED(0, "禁用"), ENABLED(1, "启用");
publicintvalue;private String description;
privateUserStatus(intvalue, String description) {this.value = value;this.description = description; }
public String getDescription() {return description; }
publicvoidsetDescription(String description) {this.description = description; }}

正例：

publicenum UserStatus {
DISABLED(0,"禁用"),
ENABLED(1,"启用");

privatefinalint value;
privatefinal String description;

privateUserStatus(int value, String description){
this.value = value;
this.description = description;
}

publicintgetValue(){
return value;
}

public String getDescription(){
return description;
}
}

"a.ab.abc".split(".");// 结果为[]
"a|ab|abc".split("|");// 结果为["a", "|", "a", "b", "|", "a", "b", "c"]

"a.ab.abc".split("\\.");// 结果为["a", "ab", "abc"]
"a|ab|abc".split("\\|");// 结果为["a", "ab", "abc"]

总结

这篇文章，可以说是从事 Java 开发的经验总结，分享出来以供大家参考。希望能帮大家避免踩坑，让代码更加高效优雅。

王超，花名麟超，阿里巴巴高级地图技术工程师，一直从事Java研发相关工作。Github id: starcwang

本文缩略图：icon by 訫诺-好逑

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