Java基础面试题--包装类型与基本数据类型(String、Integer)
public class TestInteger {
public static void main(String[] args) {
//Integer面试题
Integer n1 = 1;
Integer m1 = new Integer(1);
Integer c1 = Integer.valueOf(1);
Integer n2 = 127;
Integer m2 = new Integer(127);
Integer c2 = Integer.valueOf(127);
Integer n3 = 128;
Integer m3 = new Integer(128);
Integer c3 = Integer.valueOf(128);
System.out.println(n1 == m1); //false
System.out.println(n1 == c1); //true
System.out.println(m1 == c1); //false
System.out.println(n2 == m2); //false
System.out.println(n2 == c2); //true
System.out.println(m2 == c2); //false
System.out.println(n3 == m3); //false
System.out.println(n3 == c3); //false
System.out.println(m3 == c3); //false
//String面试题
final String MESSAGE="taobao";
String a ="tao"+"bao";
String b="tao";
String c="bao";
System.out.println(a==MESSAGE); //true
System.out.println((b+c)==MESSAGE); //false
}
}
Integer num1 = new Integer(100);
Integer num2 = new Integer(100);
System.out.println(num1 == num2); //包装类型存放在堆中，内存地址指向不同
System.out.println(num1.equals(num2)); //只判断值是否相同
Integer num3 = Integer.valueOf(100);
System.out.println(num1 == num3); //false
Integer num4 = Integer.valueOf(100);
System.out.println(num4 == num3); //true
Integer num5 = Integer.valueOf(200);
Integer num6 = Integer.valueOf(200);
System.out.println(num5 == num6); //false
Integer源码：
//缓存基本数据类型代码
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
valueOf(String s)和valueOf(int i)两个方法基本相同,此方法将始终缓存-128到127范围内的值，如果i的值在-128至127之间，则返回基本数据类型,不创建Integer对象,基本数据类型数据保存在JVM栈中,如果不在缓存返回则创建Integer对象.
String intern方法(返回常量池中该字符串的引用)：
/**
* Returns a canonical representation for the string object.
* <p>
* A pool of strings, initially empty, is maintained privately by the
* class {@code String}.
* <p>
* When the intern method is invoked, if the pool already contains a
* string equal to this {@code String} object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this {@code String} object is added to the
* pool and a reference to this {@code String} object is returned.
* <p>
* It follows that for any two strings {@code s} and {@code t},
* {@code s.intern() == t.intern()} is {@code true}
* if and only if {@code s.equals(t)} is {@code true}.
* <p>
* All literal strings and string-valued constant expressions are
* interned. String literals are defined in section 3.10.5 of the
* <cite>The Java™ Language Specification</cite>.
*
* @return a string that has the same contents as this string, but is
* guaranteed to be from a pool of unique strings.
*/
public native String intern();
String mxh1 = "mxh".intern(); String mxh0 = "mxh"; System.out.println(mxh0 == mxh1); //true
当常量池中不存在"mxh"这个字符串的引用,则将这个对象的引用加入常量池,返回则个对象的引用;
当常量池中存在"mxh"这个字符串的引用,返回这个对象的引用.
"mxh".intern()在常量池中创建"mxh"的引用,mxh0引用了常量池中的"mxh",故结果为true.
包装类型与基本数据类型的区别:
1. 包装类型允许为null,基本数据类型不可以;
2. 包装类型存放在堆中,基本数据类型存放在栈中;
3. 从效率上将,基本数据类型效率高于包装类型;
4. 包装类型可以做泛型，基本数据类型不可以；
问题解析：
1.为什么包装类型允许为null，而基本数据类型不可以？
基本数据类型存放在栈内存中，包装类型及对象存放在对内存中，栈内存中存放的是对象的引用地址，包装类型可以没有指向堆的内存地址，即对象可以为null；
2.为什么效率上基本数据类型比包装类型效率高？
基本数据类型存放在栈中，而包装类型存放在堆中，在栈中找到内存地址后还需要去堆中取值，多了一步，所以基本数据类型效率高；
3.为什么包装类型可以做泛型而基本数据类型不可以？
集合中存放的都是Object类型及Object的子类，基本数据类型不是Object的子类，当然基本数据类型有对应的包装类；
