if默认是true？举例

在很多编程语言中，if默认是true。这意味着如果没有指定一个条件表达式，if语句就会被视为真，执行其中的代码块。下面是一个示例：

if (1) {
  console.log('这条语句会被执行');
}

在这个例子中，if语句的条件表达式是1。尽管这不是一个显式的布尔值，但它会被隐式地转换为true，因此if语句中的代码块会被执行。

相关问题

c语言播放手机安卓系统默认铃声的代码举例？

以下是一个使用C语言在Android系统上播放默认铃声的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <jni.h>
#include <android/log.h>
#include <android/native_activity.h>
#include <android/native_window.h>
#include <android/native_window_jni.h>
#include <SLES/OpenSLES.h>
#include <SLES/OpenSLES_Android.h>

#define LOG_TAG "RingtonePlayer"
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__)

//声明OpenSLES会话接口对象、音频播放器对象和回调接口
SLObjectItf engineObject;
SLEngineItf engineEngine;
SLObjectItf outputMixObject;
SLObjectItf audioPlayerObject;
SLPlayItf audioPlayerPlay;
SLAndroidSimpleBufferQueueItf audioPlayerBufferQueue;

//声明默认铃声ID和缓存大小
static const char defaultRingtoneUri[] = "content://settings/system/ringtone";
short buffer[4096];

//声明C函数，用于从Java代码调用
JNIEXPORT void JNICALL Java_com_example_ringtoneplayer_RingtonePlayer_playRingtone(JNIEnv* env, jobject thiz)
{
SLresult result;

//初始化OpenSLES引擎
result = slCreateEngine(&engineObject, 0, NULL, 0, NULL, NULL);
result = (*engineObject)->Realize(engineObject, SL_BOOLEAN_FALSE);
result = (*engineObject)->GetInterface(engineObject, SL_IID_ENGINE, &engineEngine);

//初始化混音器对象
result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 0, NULL, NULL);
result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);

//初始化音频播放器对象
SLDataLocator_AndroidSimpleBufferQueue loc_bufq = { SL_DATALOCATOR_ANDROIDSIMPLEBUFFERQUEUE, 2 };
SLDataFormat_PCM format_pcm = { SL_DATAFORMAT_PCM, 1, SL_SAMPLINGRATE_44_1, SL_PCMSAMPLEFORMAT_FIXED_16, SL_PCMSAMPLEFORMAT_FIXED_16, SL_SPEAKER_FRONT_CENTER, SL_BYTEORDER_LITTLEENDIAN };
SLDataSource audioSrc = { &loc_bufq, &format_pcm };
SLDataLocator_OutputMix loc_outmix = { SL_DATALOCATOR_OUTPUTMIX, outputMixObject };
SLDataSink audioSnk = { &loc_outmix, NULL };
const SLInterfaceID ids[1] = { SL_IID_BUFFERQUEUE };
const SLboolean req[1] = { SL_BOOLEAN_TRUE };
result = (*engineEngine)->CreateAudioPlayer(engineEngine, &audioPlayerObject, &audioSrc, &audioSnk, 1, ids, req);
result = (*audioPlayerObject)->Realize(audioPlayerObject, SL_BOOLEAN_FALSE);
result = (*audioPlayerObject)->GetInterface(audioPlayerObject, SL_IID_PLAY, &audioPlayerPlay);
result = (*audioPlayerObject)->GetInterface(audioPlayerObject, SL_IID_BUFFERQUEUE, &audioPlayerBufferQueue);

//打开默认铃声文件
jclass contextClass = (*env)->FindClass(env, "android/content/Context");
jfieldID systemFieldID = (*env)->GetStaticFieldID(env, contextClass, "SYSTEM_SERVICE", "Ljava/lang/String;");
jstring stringObj = (*env)->NewStringUTF(env, "audio");
jobject audioManagerObj = (*env)->CallObjectMethod(env, thiz, (*env)->GetStaticMethodID(env, contextClass, "getSystemService", "(Ljava/lang/String;)Ljava/lang/Object;"), stringObj);
jclass audioManagerClass = (*env)->GetObjectClass(env, audioManagerObj);
jmethodID getRingtoneURIID = (*env)->GetMethodID(env, audioManagerClass, "getRingtoneUri", "(I)Landroid/net/Uri;");
jobject ringtoneURI = (*env)->CallObjectMethod(env, audioManagerObj, getRingtoneURIID, 1);
jstring ringtoneUriString = (jstring)(*env)->CallObjectMethod(env, ringtoneURI, (*env)->GetMethodID(env, (*env)->FindClass(env, "android/net/Uri"), "toString", "()Ljava/lang/String;"));
const char* ringtonePath = (*env)->GetStringUTFChars(env, ringtoneUriString, 0);

//打开音频文件，并将缓存数据填充到音频播放器对象的缓存队列
FILE* file = fopen(ringtonePath, "rb");
if (file == NULL) {
LOGE("Failed to open file");
return;
}
fseek(file, sizeof(short) * 44, SEEK_SET);
while (1) {
result = fread(buffer, sizeof(short), 4096 / sizeof(short), file);
if (result < 1) {
break;
}
result = (*audioPlayerBufferQueue)->Enqueue(audioPlayerBufferQueue, buffer, result * sizeof(short));
}

//开始播放音频
result = (*audioPlayerPlay)->SetPlayState(audioPlayerPlay, SL_PLAYSTATE_PLAYING);
result = (*audioPlayerBufferQueue)->RegisterCallback(audioPlayerBufferQueue, playerCallback, NULL);
result = (*audioPlayerBufferQueue)->Enqueue(audioPlayerBufferQueue, buffer, 0);
}

//回调函数，用于处理缓存队列已被耗尽的情况
void playerCallback(SLAndroidSimpleBufferQueueItf queue, void* context)
{
}

java里的methods to string和methods equals是什么，举例说明

在Java中，每个类都有一个默认的`toString()`方法和`equals()`方法，如果不进行重写，那么它们将使用默认实现。

`toString()`方法是将对象转换为字符串的方法。默认实现返回一个包含类名和哈希码的字符串。如果要自定义`toString()`方法，可以在类中重写该方法，以返回你期望的字符串。

举个例子：

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public static void main(String[] args) {
        Person person = new Person("张三", 20);
        System.out.println(person); // 输出：Person{name='张三', age=20}
    }
}

`equals()`方法用于比较两个对象是否相等。默认实现比较的是两个对象的引用是否相等。如果要自定义`equals()`方法，可以在类中重写该方法，以实现你期望的比较逻辑。

举个例子：

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        Person person = (Person) obj;
        return age == person.age &&
                Objects.equals(name, person.name);
    }

    public static void main(String[] args) {
        Person person1 = new Person("张三", 20);
        Person person2 = new Person("张三", 20);
        System.out.println(person1.equals(person2)); // 输出：true
    }
}

在这个例子中，我们通过重写`equals()`方法，实现了比较两个`Person`对象的`name`和`age`属性是否相等。