Effective Java 英文 第二版 读书笔记 Item 5:Avoid creating unnecessary objects.

It is often appropriate to reuse a single object instead of creating a new functionally equivalent object each time it is needed.Reuse can be both faster and more stylish.An object can always be reused if it is immutable.

String s=new String(“no”);  //DON”T DO THIS!

The statement creates a new String instance each time it is executed.

String s=”yes”;

This version uses a single String instance,avoid creating unnecessary objects. 

package creatObjects;

import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class Person {
    private final Date birthDate=new Date();
    // Other fields,methods,and constructor omitted

    /*
     * The starting and ending dates of the baby boom.
     */
    private static final Date BOOM_START;
    private static final Date BOOM_END;

    //condition: this class must be invoke,otherwise it will direct  poor performance 
    static {
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
        // set(int year, int month, int date, int hourOfDay, int minute, int
        // second)
        BOOM_START = gmtCal.getTime();
        gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
        BOOM_END = gmtCal.getTime();
    }

    public boolean isBabyBoomer() {
        return birthDate.compareTo(BOOM_START) >= 0
                && birthDate.compareTo(BOOM_END) < 0;
    }
}

There’s a new way to create unnecessary objects in release 1.5.It is called autoboxing,and it allows the programmer to mix primitive and boxed primitive types,boxing and unboxing automatically as needed.

Prefer primitives to boxed primitives,and watch out for unintentional autoboxing.

Don’t create a new object when you should reuse an existing one.

Don’t reuse an existing object when you should create a new one.