Preconditions in Google Guava

Preconditions: Make the precondition judgment of method calls easier.
Guava provides several utility methods for precondition judgment in the Preconditions class, and we recommend [statically importing these methods in Eclipse] three variants of each method:

The checkArgument() method is used to check whether the incoming value is true.

boolean flag=false;
checkArgument(flag);

operation result:

Exception in thread "main" java.lang.IllegalArgumentException
    at com.google.common.base.Preconditions.checkArgument(Preconditions.java:108)
    at guavaDemo.Test02.main(Test02.java:9)

Of course, this method has many overloaded methods, here we introduce a demonstration:

int max=1,min=2;//我们期待max是大于min的    
checkArgument(max>min,"max的值小于min的值");

operation result:

Exception in thread "main" java.lang.IllegalArgumentException: max的值小于min的值
   at com.google.common.base.Preconditions.checkArgument(Preconditions.java:122)
   at guavaDemo.Test02.main(Test02.java:12)

The checkNotNull(T) method is used to check whether the value of T is null.

String str=null; 
checkNotNull(str);

operation result:

Exception in thread "main" java.lang.NullPointerException
    at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:770)
    at guavaDemo.Test02.main(Test02.java:15)

checkState(boolean) Check the state of the object.

    String str=null;
    checkState(str.isEmpty());

operation result:

Exception in thread "main" java.lang.NullPointerException
    at guavaDemo.Test02.main(Test02.java:17)

checkElementIndex(int index, int size), check whether the index value of a list, string, or array is legal.

int[] arr=new int[5];
checkElementIndex(5, arr.length);

operation result:

Exception in thread "main" java.lang.IndexOutOfBoundsException: index (5) must be less than size (5)
    at com.google.common.base.Preconditions.checkElementIndex(Preconditions.java:1177)
    at com.google.common.base.Preconditions.checkElementIndex(Preconditions.java:1159)
    at guavaDemo.Test02.main(Test02.java:20)

checkPositionIndex(int index, int size), check whether the position is valid, the following example still uses the array defined in the above example.
checkPositionIndex(5, arr.length);The 5 position exists and works fine.
checkPositionIndex(6, arr.length);6 position does not exist, throws an exception.

Exception in thread "main" java.lang.IndexOutOfBoundsException: index (6) must not be greater than size (5)
    at com.google.common.base.Preconditions.checkPositionIndex(Preconditions.java:1222)
    at com.google.common.base.Preconditions.checkPositionIndex(Preconditions.java:1204)
    at guavaDemo.Test02.main(Test02.java:22)

checkPositionIndexes(int start, int end, int size), check if a range is valid.
checkPositionIndexes(3, 6, arr.length);
operation result:

Exception in thread "main" java.lang.IndexOutOfBoundsException: index (6) must not be greater than size (5)
    at com.google.common.base.Preconditions.checkPositionIndex(Preconditions.java:1222)
    at com.google.common.base.Preconditions.checkPositionIndex(Preconditions.java:1204)
    at guavaDemo.Test02.main(Test02.java:22)

The above are some of the most basic precondition checking methods provided to us in guava.

Next, let's take a look at the equals method and hashcode method provided by guava. The code is relatively simple and will not be explained in detail here.

        System.out.println(Objects.equal(null, 'a'));
        System.out.println(Objects.equal(null, null));
        System.out.println(Objects.equal('a', null));
        System.out.println(Objects.equal('a','a'));
        
        String str1="zhaotong1";
        System.out.println(Objects.hashCode(str1));

Results of the:

false
true
false
true
-1420540160

Preconditions in Google Guava

Guess you like