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标题: 今天的晚上的API考试第三题为什么选A啊？ [打印本页]
作者: 花陌相惜 时间: 2016-7-30 22:14
标题: 今天的晚上的API考试第三题为什么选A啊？
public static void main(String[] args){
String s1 = “abc”;
String s2 = “xyz”;
show(s1,s2);
System.out.println(s1+”-----”+s2);
}
static void show(String s1,String s2){
s1 = s2+s1+”Q”; 答案解释//show方法中的s1= “xyzabcQ”但是main函数中s1没有变化.
s2 = “W”+s1; //原理同上.
}
a) abc-----xyz
b) xyzabcQ-----xyzWabc
c) xyzabcQ---- xyzabcQWabc
d) xyzQ----Wabc
我记得如果那两个数为基本数据类型的话
int a=10;
int b=20;
show(int a,int b);
System.out.println(a+”-----”+b);
这里打印的a,b是10,20，但是上面哪个不是引用数据类型吗？为什么打印出的值没有变啊？
作者: Oh_JAVA 时间: 2016-7-30 22:28
我也帮你回答一题..O(∩_∩)O哈哈~
JDK文档中,String类每一个看起来会修改String值的方法,实际上都创建了一个全新的String对象,以包含修改后的字符串内容.最初的String对象则丝毫未动
每当把String对象当做方法的参数时,都会复制一份引用
-----来自JAVA编程思想
作者: 花陌相惜 时间: 2016-7-30 22:36
嗯嗯 看来还是要多看API帮助文档
作者: chenlongwen 时间: 2016-7-30 22:39
今天老师不高兴了
作者: Kaweh. 时间: 2016-7-30 22:41
注意 输出语句没有在show方法里边，因为字符串是常量，进show方法里 并没有改变实参的值
作者: Kingdragon 时间: 2016-7-30 22:45
String是特殊的类 完全可以把这种方式的String s = “adsaf”； 看成是常量 {存在于常量池中} 因为是final修饰
所以不会改变的 做连接时 相当于是在常量池中新创建了一个新的字符串
作者: 花陌相惜 时间: 2016-7-30 22:53
我知道String s = “adsaf”是存在于常量池中，final修饰没发现啊。。哪里有显示啊！
作者: Kingdragon 时间: 2016-7-30 22:56
你去看源码 String 中对字符串的操作实际是对字符数组的操作
作者: anywei 时间: 2016-7-30 22:59
这问题俺也是不会 有大神说String作为参数传递和基本数据类型一样 传递的是值 不是地址值
作者: Kingdragon 时间: 2016-7-30 23:00
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
/** Cache the hash code for the string */
private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
public String() {
this.value = new char[0];
}
/**
* Initializes a newly created {@code String} object so that it represents
* the same sequence of characters as the argument; in other words, the
* newly created string is a copy of the argument string. Unless an
* explicit copy of {@code original} is needed, use of this constructor is
* unnecessary since Strings are immutable.
*
* @param original
* A {@code String}
*/
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
/**
* Allocates a new {@code String} so that it represents the sequence of
* characters currently contained in the character array argument. The
* contents of the character array are copied; subsequent modification of
* the character array does not affect the newly created string.
*
* @param value
* The initial value of the string
*/
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
/**
* Allocates a new {@code String} that contains characters from a subarray
* of the character array argument. The {@code offset} argument is the
* index of the first character of the subarray and the {@code count}
* argument specifies the length of the subarray. The contents of the
* subarray are copied; subsequent modification of the character array does
* not affect the newly created string.
*
* @param value
* Array that is the source of characters
*
* @param offset
* The initial offset
*
* @param count
* The length
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and {@code count} arguments index
* characters outside the bounds of the {@code value} array
*/
public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
// Note: offset or count might be near -1>>>1.
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
this.value = Arrays.copyOfRange(value, offset, offset+count);
}
/**
* Allocates a new {@code String} that contains characters from a subarray
* of the <a href="Character.html#unicode">Unicode code point</a> array
* argument. The {@code offset} argument is the index of the first code
* point of the subarray and the {@code count} argument specifies the
* length of the subarray. The contents of the subarray are converted to
* {@code char}s; subsequent modification of the {@code int} array does not
* affect the newly created string.
*
* @param codePoints
* Array that is the source of Unicode code points
*
* @param offset
* The initial offset
*
* @param count
* The length
*
* @throws IllegalArgumentException
* If any invalid Unicode code point is found in {@code
* codePoints}
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and {@code count} arguments index
* characters outside the bounds of the {@code codePoints} array
*
* @since 1.5
*/
public String(int[] codePoints, int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
// Note: offset or count might be near -1>>>1.
if (offset > codePoints.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
final int end = offset + count;
// Pass 1: Compute precise size of char[]
int n = count;
for (int i = offset; i < end; i++) {
int c = codePoints;
if (Character.isBmpCodePoint(c))
continue;
else if (Character.isValidCodePoint(c))
n++;
else throw new IllegalArgumentException(Integer.toString(c));
}
// Pass 2: Allocate and fill in char[]
final char[] v = new char[n];
for (int i = offset, j = 0; i < end; i++, j++) {
int c = codePoints;
if (Character.isBmpCodePoint(c))
v[j] = (char)c;
else
Character.toSurrogates(c, v, j++);
}
this.value = v;
}
这是部分源码
字段和构造方法
作者: 花陌相惜 时间: 2016-7-30 23:01
嗯呢 看到了 多谢
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
/** Cache the hash code for the string */
private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
/**
作者: lynch123 时间: 2016-7-30 23:10
学到了，有空还的多看源码。
作者: HeiMa2Wangkunpe 时间: 2016-7-31 00:42
字符串虽然是引用数据类型 但是作为实际参数传递 其值不会发生改变
作者: 花陌相惜 时间: 2016-7-31 10:38
实际参数传递不是基本数据类型吗？
作者: 东东瑞 时间: 2016-7-31 10:40
有回复的好的
作者: 花陌相惜 时间: 2016-7-31 10:42
我去 你就进来瞄瞄
作者: 18072727834 时间: 2016-8-1 21:30
多看源码 好处多多
作者: anyeyyc 时间: 2016-8-1 22:48
String 类型直接初始化的值存在常量池中
new出来的对象存在堆内存中
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