我手头有一个 Java 原始类型：
Class<?> c = int.class; // or long.class, or boolean.class
我想为这个类获取一个默认值——具体来说，如果它们没有被初始化，这个值就会被分配给这种类型的字段。例如，0对于数字，false对于布尔值。
有没有通用的方法来做到这一点？我试过这个：
c.newInstance()
但我得到的是一个InstantiationException，而不是一个默认实例。
问问题
41487 次
9 回答
57
番石榴库已经包含： http:
//guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/base/Defaults.html
调用defaultValue将为任何原始类型（由 JLS 指定）返回默认值，为任何其他类型返回 null。
像这样使用它：
import com.google.common.base.Defaults;
Defaults.defaultValue(Integer.TYPE); //will return 0
于 2010-05-23T16:32:18.927 回答
25
通过创建一个元素的数组并检索其第一个值，可以获得任何类型的默认值。
private static <T> T getDefaultValue(Class<T> clazz) {
return (T) Array.get(Array.newInstance(clazz, 1), 0);
}
这样就不需要考虑所有可能的原始类型，而创建单元素数组的成本通常可以忽略不计。
于 2016-07-07T10:30:04.650 回答
15
这就是我的想法（虽然没有通过优雅测试）：
public class PrimitiveDefaults {
// These gets initialized to their default values
private static boolean DEFAULT_BOOLEAN;
private static byte DEFAULT_BYTE;
private static short DEFAULT_SHORT;
private static int DEFAULT_INT;
private static long DEFAULT_LONG;
private static float DEFAULT_FLOAT;
private static double DEFAULT_DOUBLE;
public static Object getDefaultValue(Class clazz) {
if (clazz.equals(boolean.class)) {
return DEFAULT_BOOLEAN;
} else if (clazz.equals(byte.class)) {
return DEFAULT_BYTE;
} else if (clazz.equals(short.class)) {
return DEFAULT_SHORT;
} else if (clazz.equals(int.class)) {
return DEFAULT_INT;
} else if (clazz.equals(long.class)) {
return DEFAULT_LONG;
} else if (clazz.equals(float.class)) {
return DEFAULT_FLOAT;
} else if (clazz.equals(double.class)) {
return DEFAULT_DOUBLE;
} else {
throw new IllegalArgumentException(
"Class type " + clazz + " not supported");
}
}
}
于 2010-05-23T14:18:08.460 回答
7
Guava 的替代方案Defaults.java，它可以让实现找出默认值（通过使用Antag99 的答案改进）：
import static java.util.stream.Collectors.toMap;
import java.lang.reflect.Array;
import java.util.Map;
import java.util.stream.Stream;
public class DefaultValue {
/**
* @param clazz
* the class for which a default value is needed
* @return A reasonable default value for the given class (the boxed default
* value for primitives, <code>null</code> otherwise).
*/
@SuppressWarnings("unchecked")
public static <T> T forClass(Class<T> clazz) {
return (T) DEFAULT_VALUES.get(clazz);
}
private static final Map<Class<?>, Object> DEFAULT_VALUES = Stream
.of(boolean.class, byte.class, char.class, double.class, float.class, int.class, long.class, short.class)
.collect(toMap(clazz -> (Class<?>) clazz, clazz -> Array.get(Array.newInstance(clazz, 1), 0)));
public static void main(String... args) {
System.out.println(DefaultValue.forClass(int.class)); // 0
System.out.println(DefaultValue.forClass(Integer.class)); // null
}
}
于 2014-03-07T15:22:00.673 回答
2
您可以通过反射来做到这一点，但写出来是最简单和最清晰的，例如
Object defaultValue(Class cls)
{
Map defaults = new HashMap();
defaults.put(Integer.TYPE, Integer.valueOf(0));
defaults.put(Double.TYPE, Double.valueOf(0));
defaults.put(Boolean.TYPE, Boolean.FALSE);
//... etc
return defaults.get(cls);
}
当然，您可能希望将映射初始化移到构造函数或类似的构造器中，以进行一次初始化。
相当简洁——它很优雅吗？
于 2010-05-23T14:03:02.707 回答
1
没有一种优雅的方法可以做到这一点。事实上，甚至不可能声明将返回原始值本身的方法的签名。
你能来的最接近的是这样的：
public Object defaultValue(Class cls) {
if (class == Boolean.TYPE) {
return Boolean.FALSE;
} else if (class == Byte.TYPE) {
return Byte.valueOf(0);
} else if (class == Short.TYPE) {
...
} else {
return null;
}
}
于 2010-05-23T13:56:26.200 回答
0
原语的类变量不需要初始化或设置默认值。但是，必须初始化在其他范围内声明的变量，否则会出现编译错误。
public class PrimitiveStuff {
private int aInt;
private long aLong;
private boolean aBoolean;
public PrimitiveStuff() {
System.out.println("aInt : " + aInt); //prints 0
System.out.println("aLong: "+ aLong);//prints 0
System.out.println("aBoolean: " + aBoolean);//prints false
}
public void doStuff(){
int outherInt;
System.out.println(outherInt); //will not compile
}
public static void main(String[] args) {
new PrimitiveStuff();
}
}
于 2010-05-23T14:32:44.990 回答
0
如果您想要包含字符串数据类型的默认值，请尝试此操作：
private static Object getDefaultValue(Field field) {
if (Number.class.isAssignableFrom(field.getType())) {
if (Double.class.isAssignableFrom(field.getType())
|| Float.class.isAssignableFrom(field.getType())) {
return 0.0;
} else {
return 0;
}
} else if (Boolean.class.isAssignableFrom(field.getType())) {
return false;
} else if (Timestamp.class.isAssignableFrom(field.getType())) {
return new Timestamp(Instant.now().toEpochMilli());
} else {
return "NULL";
}
}
于 2020-07-31T15:33:30.317 回答
-1
根据Jack Leow 的回答，我创建了这个类：
/**
<P>{@code java InitializedObjectUtil}</P>
**/
public class InitializedObjectUtil {
public static final void main(String[] igno_red) {
printDefault("boolean");
printDefault("char");
printDefault("byte");
printDefault("short");
printDefault("int");
printDefault("long");
printDefault("float");
printDefault("double");
printDefault("java.lang.AnythingAndEverythingElse");
}
private static final void printDefault(String s_type) {
Object oDflt = InitializedObjectUtil.getForClassName(s_type);
System.out.println(s_type + " default is \"" + oDflt + "\"");
}
/**
<P>The default value for a boolean is {@code false}.</P>
<P>Viewed 1/21/2014
<BR><CODE><A HREF="http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html">http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html</A></CODE>:</P>
<P><B>Default Values:</B> </P>
<P>It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style. The following chart summarizes the default values for the above data types.</P>
<PRE>{@literal
Data Type Default Value (for fields)
--------------------------------------
byte 0
short 0
int 0
long 0L
float 0.0f
double 0.0d
char '\u0000'
String (or any object) null
boolean false}</PRE>
@see #getForClass(String) getForClass(s)
@see #getForClassName(String) getForClassName(s)
@see #DEFAULT_CHAR
@see #DEFAULT_BYTE
@see #DEFAULT_SHORT
@see #DEFAULT_INT
@see #DEFAULT_LONG
@see #DEFAULT_FLOAT
@see #DEFAULT_DOUBLE
**/
public static final Boolean DEFAULT_BOOLEAN = false;
/**
<P>The default value for a char {@code '\u0000'}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Character DEFAULT_CHAR = '\u0000';
/**
<P>The default value for a byte is {@code 0}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Byte DEFAULT_BYTE = 0;
/**
<P>The default value for a short is {@code 0}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Short DEFAULT_SHORT = 0;
/**
<P>The default value for a int is {@code 0}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Integer DEFAULT_INT = 0;
/**
<P>The default value for a long is {@code 0L}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Long DEFAULT_LONG = 0L;
/**
<P>The default value for a float {@code 0.0f}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Float DEFAULT_FLOAT = 0.0f;
/**
<P>The default value for a double {@code 0.0d}.</P>
@see #DEFAULT_BOOLEAN
**/
public static final Double DEFAULT_DOUBLE = 0.0d;
/**
<P>Get an object containing an initialized value for the static class-type.</P>
@param cls_static May not be {@code null}.
@return <CODE>{@link getForClassName(String) getForClassName}(cls_static.getName())</CODE>
**/
public static final Object getForClass(Class cls_static) {
try {
return getForClassName(cls_static.getName());
} catch(RuntimeException rtx) {
throw new NullPointerException("getForClass: cls_static");
}
}
/**
<P>Get an object containing an initialized value for the type whose name is in a string.</P>
<P>Idea from (viewed 1/2/2014)
<BR> {@code <A HREF="https://stackoverflow.com/questions/2891970/getting-default-value-for-java-primitive-types/2892067#2892067">https://stackoverflow.com/questions/2891970/getting-default-value-for-java-primitive-types/2892067#2892067</A>}</P>
@param s_type May not be {@code null}.
@return If {@code s_type} is equal to<UL>
<LI>{@code "boolean"}: {@link #DEFAULT_BOOLEAN}</LI>
<LI>{@code "char"}: {@link #DEFAULT_CHAR}</LI>
<LI>{@code "byte"}: {@link #DEFAULT_BYTE}</LI>
<LI>{@code "short"}: {@link #DEFAULT_SHORT}</LI>
<LI>{@code "int"}: {@link #DEFAULT_INT}</LI>
<LI>{@code "long"}: {@link #DEFAULT_LONG}</LI>
<LI>{@code "float"}: {@link #DEFAULT_FLOAT}</LI>
<LI>{@code "double"}: {@link #DEFAULT_DOUBLE}</LI>
<LI><I>anything else</I>: {@code null}</LI>
</UL>
@see #getForClass(Class) getForClass(cls)
**/
public static final Object getForClassName(String s_type) {
try {
if(s_type.equals("boolean")) {
return DEFAULT_BOOLEAN;
}
} catch(NullPointerException npx) {
throw new NullPointerException("getForClassName: s_type");
}
if(s_type.equals("char")) {
return DEFAULT_CHAR;
}
if(s_type.equals("byte")) {
return DEFAULT_BYTE;
}
if(s_type.equals("short")) {
return DEFAULT_SHORT;
}
if(s_type.equals("int")) {
return DEFAULT_INT;
}
if(s_type.equals("long")) {
return DEFAULT_LONG;
}
if(s_type.equals("float")) {
return DEFAULT_FLOAT;
}
if(s_type.equals("double")) {
return DEFAULT_DOUBLE;
}
//Non-primitive type
return null;
}
}
于 2014-01-21T17:31:14.417 回答