面向对象：

ü 构造器参数太多怎么办（5个以上）？

使用构造者模式（build），

package com.xiangxue.ch04.builder01;public class FoodBuilder {//requiredprivate final String foodName;private final int reilang;//optionalprivate  int danbz;private  int dianfen;private  int zf;private  int tang;private   int wss;public static class Builder{//requiredprivate final String foodName;private final int reilang;//optionalprivate  int danbz;private  int dianfen;private  int zf;private  int tang;private   int wss;public Builder(String foodName, int reilang) {super();this.foodName = foodName;this.reilang = reilang;}public Builder danbz(int val) {this.danbz = val;return this;}//.......public FoodBuilder build() {return new FoodBuilder(this);}}private FoodBuilder(Builder builder) {foodName = builder.foodName;reilang = builder.reilang;danbz = builder.danbz;//.....}public static void main(String[] args) {FoodBuilder foodBuilder = new FoodBuilder.Builder("food2", 1000).danbz(100)//......build();}
}

抽象构造：

package com.xiangxue.ch04.builder01.buildpattern;/***类说明：客户端*/
public class Mingyun {public static void main(String[] args) {System.out.println("create NvWa");NvWa nvwa =  new NvWa();nvwa.buildPerson(new ManBuilder());nvwa.buildPerson(new WomanBuilder());}
}

package com.xiangxue.ch04.builder01.buildpattern;import com.xiangxue.ch04.builder01.buildpattern.product.Person;/***类说明：导演者*/
public class NvWa {public Person buildPerson(PersonBuilder pb) {pb.buildHead();pb.buildBody();pb.buildFoot();return pb.createPerson();}}

package com.xiangxue.ch04.builder01.buildpattern;import com.xiangxue.ch04.builder01.buildpattern.product.Man;
import com.xiangxue.ch04.builder01.buildpattern.product.Person;/***类说明：具体建造者*/
public class ManBuilder extends PersonBuilder {private Person person;public ManBuilder() {this.person = new Man();}@Overridepublic void buildHead() {person.setHead("Brave Head");}@Overridepublic void buildBody() {person.setBody("Strong body");}@Overridepublic void buildFoot() {person.setFoot("powful foot");}@Overridepublic Person createPerson() {return person;}
}

package com.xiangxue.ch04.builder01.buildpattern;import com.xiangxue.ch04.builder01.buildpattern.product.Person;/***类说明：抽象建造者*/
public abstract class PersonBuilder {//建造部件public abstract void buildHead();public abstract void buildBody();public abstract void buildFoot();public abstract Person createPerson();}

package com.xiangxue.ch04.builder01.buildpattern;import com.xiangxue.ch04.builder01.buildpattern.product.Person;
import com.xiangxue.ch04.builder01.buildpattern.product.Woman;/***类说明：具体建造者*/
public class WomanBuilder extends PersonBuilder {private Person person;public WomanBuilder() {this.person = new Woman();}@Overridepublic void buildHead() {person.setHead("Pretty Head");}@Overridepublic void buildBody() {person.setBody("soft body");}@Overridepublic void buildFoot() {person.setFoot("long white foot");}@Overridepublic Person createPerson() {return person;}}

package com.xiangxue.ch04.builder01.buildpattern.product;/***类说明：具体的产品*/
public class Man extends Person {public Man() {System.out.println("create a man");}@Overridepublic String toString() {return "Man{}";}
}

package com.xiangxue.ch04.builder01.buildpattern.product;/***类说明：产品类，抽象的*/
public abstract class Person {protected String head;protected String body;protected String foot;public String getHead() {return head;}public void setHead(String head) {this.head = head;}public String getBody() {return body;}public void setBody(String body) {this.body = body;}public String getFoot() {return foot;}public void setFoot(String foot) {this.foot = foot;}
}

package com.xiangxue.ch04.builder01.buildpattern.product;/***类说明：具体的产品*/
public class Woman extends Person {public Woman() {System.out.println("create a Woman");}@Overridepublic String toString() {return "Woman{}";}
}

不需要实例化的类应该构造器私有

如：工具类

不要创建不必要的对象

package com.xiangxue.ch04.createobj03;public class Sum {public static void main(String[] args) {long start = System.currentTimeMillis();Long sum = 0L;//对象for(long i=0;i<Integer.MAX_VALUE;i++) {sum = sum+i;//new 20多亿的Long的实例}System.out.println("spend time:"+(System.currentTimeMillis()-start)+"ms");}
}

Long sum = 0L;改为long sum时，花费时间就很少，因为每次sum = sum+i;时就new出了一个新对象。

避免使用终结方法

Finalize方法就算代码里写了，也不一定会被jdk调用；
ü 使类和成员的可访问性最小化
ü 使可变性最小化
ü 优先使用复合

继承：

package com.xiangxue.ch04.countset07;import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;/***类说明：统计增加的元素个数，继承*/
public class ExtendsHashSet<E> extends HashSet<E> {private int addCount = 0;//计数器public ExtendsHashSet() {}@Overridepublic boolean add(E e) {addCount++;return super.add(e);}@Overridepublic boolean addAll(Collection<? extends E> c) {addCount = addCount+c.size();return super.addAll(c);}public int getAddCount() {return addCount;}public static void main(String[] args) {ExtendsHashSet<String> s = new ExtendsHashSet<String>();s.addAll(Arrays.asList("Mark","James","Lison"));System.out.println(s.getAddCount());  // 结果为6，addAll调用了add方法}
}

组合：

package com.xiangxue.ch04.countset07;import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.Set;/***类说明：统计增加的元素个数，组合*/
public class CompositionSet<E>{private int addCount = 0;private final Set<E> s;public int getAddCount() {return addCount;}public CompositionSet(Set<E> s) {super();this.s = s;}public boolean add(E e) {addCount++;return s.add(e);}public boolean addAll(Collection<? extends E> c) {addCount = addCount+c.size();return s.addAll(c);}public static void main(String[] args) {CompositionSet<String> s = new CompositionSet<>(new HashSet<String>());s.addAll(Arrays.asList("Mark","James","Lison"));System.out.println(s.getAddCount()); // 结果为3}
}

方法

可变参数要谨慎使用

可变参数允许传0个参数；
package com.xiangxue.ch04.varargs09;import java.util.Collections;import com.xiangxue.ch04.enmu12.SampleIntConst;public class VarArags {//private User user ;public static void userOper(int count) {//SampleIntConst.ORDER_DEPOT_PAID;if (count<0) {throw new IllegalArgumentException("");}}//可能很多 0~很多static int sum(int... args) {int sum = 0;for (int arg : args)sum += arg;return sum;}//要求参数的个数，是1~多个//
//  static int sum1(int... args) {//      if(args.length==0) {//          //做点异常处理
//      }
//      if(args[0]==100) {//
//      }
//      for(int i=1;i<args.length;i++) {//          int sum = 0;
//          sum += args[i];
//          return sum;
//      }
//  }
//
//  static int sum2(int flag, int... args) {//      if(flag==100) {//
//      }
//      int sum = 0;
//      for (int arg : args)
//          sum += arg;
//      return sum;
//      return Collections.EMPTY_LIST;
//  }}

返回零长度的数组或集合，不要返回null

集合可以返回这个:
return Collections.EMPTY_LIST;

优先使用标准的异常

public static void userOper(int count) {//SampleIntConst.ORDER_DEPOT_PAID;if (count<0) {throw new IllegalArgumentException("");}
}

此处，子类应该重写该方法，然后子类调用自己重写后的方法，如果子类没有重写，就会调用到父类的该方法，此时，父类抛出异常。

通用程序设计

用枚举代替int 常量

package com.xiangxue.ch04.enmu12;public class SampleIntConst {//延时订单状态public static final int ORDER_DEPOT_UNPAY = 0;//public static final String ORDER_DEPOT_UNPAY_DESC = "unpay";public static final int ORDER_DEPOT_PAID = 3;public static final int ORDER_DEPOT_TIMOUT = 2;//物流订单状态public static final int ORDER_LOGISTICS_READY = 0;public static final int ORDER_LOGISTICS_TRANSPORT = 1;public static final int ORDER_LOGISTICS_ARRIVED = 2;//public enum Depot{UNPAY,PAID,TIMOUT}//0,1,2public enum Logistics{READY,TRANSPORT,ARRIVED}public static void main(String[] args) {System.out.println(Depot.UNPAY);System.out.println(Depot.UNPAY.ordinal());
//      if(==0)
//          switch status
//          case 0:}
}

写代码时可能会写错，调用该常亮时，订单的状态常量可能使用到库存的状态常量，只是常量名有变化，如果没有仔细看，就over了。

package com.xiangxue.ch04.enmu12;public enum DepotEnum {UNPAY(0,"未支付"),PAID(1,"已支付"),TIMOUT(-1,"超时");private int status;private String desc;private String dbInfo;//其他属性private DepotEnum(int status, String desc) {this.status = status;this.desc = desc;}public int getStatus() {return status;}public String getDesc() {return desc;}public String getDbInfo() {return dbInfo;}public int calcStatus(int params) {return status+params;}public static void main(String[] args) {for(DepotEnum e:DepotEnum.values()) {System.out.println(e+":"+e.calcStatus(14));}}
}

枚举和行为绑定：

package com.xiangxue.ch04.enmu12;/***类说明：枚举和行为绑定*/
public class ActiveEnum {public enum NormalActive{PLUS,MINUS,MULTI,DIVIDS,DIFFER;double oper(double x,double y) {switch(this) {case PLUS:return x+y;case MINUS:return x-y;case MULTI:return x*y;case DIVIDS:return x/y;}throw new UnsupportedOperationException();}        }public enum BetterActive{PLUS {@Overridedouble oper(double x, double y) {return x+y;}},MINUS {@Overridedouble oper(double x, double y) {return x-y;}};abstract double oper(double x,double y);//        double oper(double x,double y) {//          switch(this) {//          case PLUS:return x+y;
//          case MINUS:return x-y;
//          case MULTI:return x*y;
//          case DIVIDS:return x/y;
//
//          //default:
//          }
//          throw new UnsupportedOperationException();
//      }       }public static void main(String[] args) {System.out.println(NormalActive.PLUS.oper(0.1, 0.2));}}

//计算加班费：
package com.xiangxue.ch04.enmu12;/***类说明：加班费计算,工作日加班2倍，节假日3倍*/
enum PayDay {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY,WUYI;private static final int HOURS_WORK = 2;private static final int HOURS_REST = 3;//超时时间double pay(double hoursOvertime) {switch(this) {case SATURDAY:case SUNDAY:return hoursOvertime*HOURS_REST;default:return hoursOvertime*HOURS_WORK;}}
}

//策略枚举：
package com.xiangxue.ch04.enmu12;
/***类说明：加班费计算,工作日加班2倍，节假日3倍*/
public enum BetterPayDay {MONDAY(PayType.WORK), TUESDAY(PayType.WORK), WEDNESDAY(PayType.WORK), THURSDAY(PayType.WORK), FRIDAY(PayType.WORK), SATURDAY(PayType.REST), SUNDAY(PayType.REST),WUYI(PayType.REST);private final PayType payType;//成员变量BetterPayDay(PayType payType) {this.payType = payType;}double pay(double hoursOvertime) {return payType.pay(hoursOvertime);}//策略枚举private enum PayType {WORK {double pay(double hoursOvertime) {return hoursOvertime*HOURS_WORK;}},REST {double pay(double hoursOvertime) {return hoursOvertime*HOURS_REST;}};private static final int HOURS_WORK = 2;private static final int HOURS_REST = 3;abstract double pay(double hoursOvertime);//抽象计算加班费的方法}public static void main(String[] args) {System.out.println(BetterPayDay.MONDAY.pay(7.5));}
}

将局部变量的作用域最小化

精确计算，避免使用float 和double

当心字符串连接的性能

package com.xiangxue.ch04.StringUnion15;public class Test {private static class Log{public static void debug(String msg){if (isDebug()) System.out.println(msg);}public static boolean isDebug(){return false;}}public static void main(String[] args) {int count = 100000000;long start = System.currentTimeMillis();
//        StringBuiler
//        StringBuffer;
//        "a"+"b"+"";for(int i = 0;i<count;i++){Log.debug("The system is running and the time is "+System.currentTimeMillis()+" now,Let's do another thing:"+System.nanoTime());}System.out.println("直接打印模式,次数："+count+"：spend time :"+(System.currentTimeMillis()-start)+"ms");start = System.currentTimeMillis();for(int i = 0;i<count;i++){if(Log.isDebug())Log.debug("The system is running and the time is "+System.currentTimeMillis()+" now,Let's do another thing:"+System.nanoTime());}System.out.println("先判断再打印模式,次数："+count+"：spend time :"+(System.currentTimeMillis()-start)+"ms");}
}

控制方法的大小

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