我手边有一个Java原型：
Class<?> c = int.class; // or long.class, or boolean.class
我想为此类获取默认值 - 具体而言，如果未初始化该值，则将该值分配给此类型的字段。例如，0表示数字，false表示布尔值。
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InstantiationException，而不是默认实例。
答案 0 :(得分：51)
番石榴图书馆已经包含：
http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/base/Defaults.html
调用defaultValue将返回任何基本类型的默认值（由JLS指定），对于任何其他类型，返回null。
像这样使用它：
import com.google.common.base.Defaults;
Defaults.defaultValue(Integer.TYPE); //will return 0
答案 1 :(得分：16)
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);
}
这种方式不需要考虑每种可能的基本类型，创建单元素数组的成本通常可以忽略不计。
答案 2 :(得分：11)
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");
}
}
}
答案 3 :(得分：7)
Guava Defaults.java的替代方案，它允许实现计算出默认值（使用Antag99’s answer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
}
}
答案 4 :(得分：2)
你可以用反射做到这一点，但写出来是最容易也是最清楚的，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，您可能希望将地图初始化移动到构造函数或类似的一次初始化。
合理简洁 - 优雅吗？
答案 5 :(得分：1)
没有一种优雅的方法可以做到这一点。实际上，甚至不可能声明将返回原始值本身的方法的签名。
你最接近的就是这样：
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;
}
}
答案 6 :(得分：0)
基元的类变量不需要初始化或使用默认值设置。但是，必须初始化其他范围中的变量声明，否则您将收到编译错误。
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();
}
}
答案 7 :(得分：0)
基于 Jack Leow's answer ，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;
}
}
答案 8 :(得分：0)
如果要使用包括String数据类型的默认值，请尝试以下操作：
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";
}
}